With 1978's "Superman vs. the Amazing Spider-Man", DC and Marvel began a series of unforgettable (90% of the time) crossovers between their biggest characters, continuing with a sequel to the aforementioned comic, plus "Batman vs. The Incredible Hulk", and "The Uncanny X-Men and the New Teen Titans". They were about to cap it off in 1983 with a crossover between their number-one teams, the Justice League of America and the Avengers, to be written by Gerry Conway and drawn by George Perez. Plans fell through, and the crossover was not to be--until 2003, when the project was revived. Perez would still draw, but the writing duties were handed over to Kurt Busiek (ironically, these two being the creative team on volume 3 of The Avengers). The result was one of the most long-awaited stories ever told, the 2003/04 four-issue mini-series "JLA/Avengers". (Fittingly, the third issue of the series used some of the original art from the proposed 1983 story.)

(Some summary info taken from the dreaded Wiki.)
"Krona, an exiled Oan, travels across and destroys universes seeking the truth of creation. When he arrives in the Marvel Universe, the Grandmaster, wanting to save his universe, proposes that they play a game. If Krona wins, the Grandmaster will lead him to a being (Galactus) in that universe who has witnessed creation. If he loses, Krona has to spare the Grandmaster’s universe. When summoning the players to participate in this game, Krona decides to swap champions, so the Grandmaster’s longtime adversaries, the Avengers, will represent Krona, and the Justice League, from Krona's home universe, will fight for the Grandmaster. This means that the Avengers must lose the game in order to save their universe." This is a game that is watched with great interest by the New God known as Metron.
The JLA are sent by the Grandmaster after the following, in addition to his universe's six objects:
The League contacts reservists Firestorm, Red Tornado, Black Canary, Green Arrow, Hawkman, Blue Beetle (Ted Kord), Zatanna, Captain Atom, and Steel to assist in the search.
The Avengers, in turn, are sent by Metron (who also gives Iron Man a Mother Box to guide them) to go after the following (in addition to his universe's six):
They bring in Ms. Marvel, She-Hulk, Jack Of Hearts, Wonder Man, Photon, and Hercules to lend a hand.
"The Justice League travels to the Marvel Universe, and are dismayed by the Avengers' failure to improve their Earth's condition. When the Avengers visit the DC Universe, they are surprised by the "futuristic" architecture of its Earth's cities and the honors that the Justice League and other native heroes receive for their deeds, and believe that the Leaguers are fascists who demand that civilians worship them. Various Leaguers and Avengers travel across the two universes and fight each other to retrieve the artifacts of power."

"A final battle for the Cosmic Cube takes place in the Marvel Universe’s Savage Land. Quicksilver claims the cosmic cube, and at that moment Krona and the Grandmaster arrive one the scene, with the latter observing and commenting that the score is even at 6-6. However, Batman and Captain America—who together investigated the cause of the contest, with help from the Atom and the Fantastic Four—arrive. Captain America throws his shield and knocks the Cube from Quicksilver's hand and into the arms of a waiting Batman. With Captain America's forfeiture of the Cosmic Cube, the Grandmaster now announces the Justice League as the victors, with the final score now being 7-5." Krona, still not wishing to concede, attacks the Grandmaster and learns the identity of Galactus, summoning him instantly and brutally attacking the devourer of worlds. The weakened Grandmaster uses the power of the artifacts and merges both universes together in an attempt to stop the chaos."

"Reality is altered such that the Justice League and Avengers are longtime allies [regularly crossing over to help one another a la the old JLA/JSA adventures], but the universes are incompatible and begin destroying each other [evidenced by the ill tension and lowered rationality between Superman and Captain America]. [The Phantom Stranger brings the assembled teams to the Grandmaster, who is looking worse for wear.] The Grandmaster [who had been left for dead by Krona] explains to the heroes that he brought the universes together to imprison Krona using the 12 items, but Krona is merging the universes further to destroy them, hoping to create a new Big Bang, which he can survive and learn the secrets of. [Krona tells Metron of his discovery that there is a force that creates universes, evidenced by a voice in one (which told Galen to not fear anything before he became Galactus) and a hand in the other (the hand of creation seen in Crisis on Infinite Earths).] Before dying the Grandmaster asks the assembled heroes to stop Krona and restore order, and shows them various events that had taken place in the separate universes. Each team member witnesses the tragedies that had befallen them in their separate universes, such as the deaths of Hal Jordan and Barry Allen. Several heroes vote to leave the universes as they are to prevent the tragedies from happening [especially the Vision and the Scarlet Witch, who fear losing their children], but Hal Jordan inspires everyone to work for the good of their worlds."

"Krona has captured the universal avatars of Eternity [MU] and Kismet [DCU]. Reality continues to change, and at Krona's base, Captain America leads every hero who has ever been a member of the Justice League or the Avengers; they battle every villain the teams have ever fought. The heroes succeed, and the universes are returned to normal with help from the Spectre, who is in fact Hal Jordan, returned to this state. Krona has imploded to form a cosmic egg, which is stored in the JLA Watchtower; Metron states that when the egg hatches, Krona will learn the secrets of its creation by being part of it. Metron and the Grandmaster discuss how Metron intentionally lured Krona to the Marvel universe. The Grandmaster says that this is the first game he played when all sides won."

Let me tell you all, just as the miniseries was a labor of love for the companies, this fancast was a long time coming for me. I love the mini, I would rank it high on my top ten favorite comic book stories. I felt it deserved to be animated, and that a DVD release wouldn't be enough--no, this needs to be SEEN by the millions, and television is the best option. Here's how I envision the result:
-The four-part miniseries would air on Cartoon Network, USA Network (or TNT, whatever), SyFy, G4--basically any network worth putting it on (except FOX).
-The voice direction would be done by Andrea Romano and Jamie Simone.
-The art and character designs would be done by Jamie McKelvie (Young Avengers, The Defenders, X-Men: Season One) in tandem with the Man of Action Studios art team.
-Before the first part of the mini, there would be a brief, live-action intro segment featuring Stan Lee and Paul Levitz.
-Some minor alterations to the character looks: instead of his 90's getup, Aquaman would wear something akin to his Young Justice/'water bearer' getup; Kyle Rayner would wear his post-2006 costume; Iron Man would wear his Bleeding Edge armor; Thor in his post-2006 getup; and Pym in his current Giant-Man suit.
*If you note the team lineups in the story, you'll find they are in certain lineups that were out-of-sync with the current issues of JLA and Avengers being put out circa 2003/04: the League is the Morrison-era team circa late 1998-99, while the Avengers lineup is the 1999-2001 group, though Justice, Firestar, and Silverclaw aren't present.
All right, enough of my yappin', it's time to get to the meat of this fancast--the voice cast! There will be new faces, there will be reprisals, and there will be the obligatory additional voices section. Presenting--
+The Justice League
-Mark Harmon reprising the voice of Superman/Clark Kent
I really thought that Harmon nailed the character in Justice League: Crisis on Two Earths, and is why I brought him up from the bench.
-Bruce Greenwood reprising the voice of Batman/Bruce Wayne
I consider Greenwood to be one of the best Batman voices in recent years (of course no disrespect to Kevin Conroy), "Under the Red Hood" proved this.
-Keri Russell reprising the voice of Wonder Woman/Diana of Themyscria
I know a lot of flak may be thrown my way, but I liked her voice for the role in the 2009 animated movie. I also thought her performance was very decent, but with this she'd get the chance to flex the character a little more.
-Kevin Sorbo as the voice of Aquaman/Orin/Arthur Curry
I seriously think that Sorbo would have the right voice for the Sea King, right up there with Scott Rummell and Phil LaMarr.
-Bret Harrison as the voice of the Flash/Wally West
Harrison I believe has been used on this site as West for live-action fancasts--but I honestly think he'd be perfect for the character vocally.
-Scott Porter as the voice of Green Lantern/Kyle Rayner
I wanted to bring Porter into another role along the lines of a character he's stated he's wanted to play--whereas it was Nova for a Marvel movie, I went with Rayner for DC, as some fans of Amalgam Comics have done their own spin on the concept, and Rayner is mixed with Rich Rider.
-Avery Brooks as the voice of Martian Manhunter/J'onn J'onzz
Like I stated before in my "Justice League: Legacy" fancast, Brooks, aside from sci-fi cred in the form of Star Trek: Deep Space Nine, has a really, REALLY good speaking voice, perfect for ol' J'onn.
-Steve Carell as the voice of Plastic Man/"Eel" O'Brian
I strongly believe Carell could pull off the wisecracking personality of Plas (along with the New Yorker accent), and he's voiced characters before, from the most minor (a guest role on Fillmore!) to the more high-profile (the lead in "Despicable Me").
+The Avengers
-Brian Bloom reprising the voice of Captain America/Steve Rogers
Here's the fourth reprisal in my cast, and of all the voices of Cap from over the decades, I think Brian Bloom's really stands out. He portrays him like a classic film hero, and of course I found it ironic that his character's squaring off against Mark Harmon's again (see what I did there...?).
-Robert Hays reprising the voice of Iron Man/Tony Stark
And here comes another unexpected return, the voice of the 1990's Iron Man himself! (Or Airplane's Ted Striker or TV's Starman, for those of you who are 80's kids.) I like Eric Loomis's portrayal from The Avengers: Earth's Mightiest, but I really like Hays's portrayal (especially in the second season of the 90's Iron Man), he brought a lot of drama to the character (without sounding like Robert Downey, Jr.).
-Eric Allan Kramer as the voice of Thor
This is probably the most obscure of the reprisals, as Kramer played a live-action (and different from the comics) version of Thor--the first actor to do so. With this, it would be a chance to see how he would handle a version of Thor more in line with the medium he sprang from.
-Linda Cardellini as the voice of the Wasp/Janet Van Dyne
I believe someone on this site used Cardellini as the Wasp in an Avengers or Ant-Man fancast, and while she would be good for a live-action take, I really think voicing her would give us a stronger portrayal of the character (without fear of losing her nerve in front of the cameras).
-David Boreanaz as the voice of Henry Pym/Giant-Man/Ant-Man/Yellowjacket/Goliath
Once again, inspiration for this pick came from his turn as Hal Jordan in "Justice League: The New Frontier", though in this case, I think he would do great with Pym, especially during the scenes where he confronts the reality that in the normal history, his and Wasp's marriage may be doomed because of him.
-Sam Witwer as the voice of Quicksilver/Pietro Maximoff
Witwer's been mostly associated with all things DC (thanks to Smallville), and of course, some users here have cast him as different Marvel characters--so count me among them. I believe Witwer could muster up an Eastern European accent for Pietro, and I think it'd be fun seeing how he bounces off of Bret Harrison's Flash. (He keeps telling Wally throughout the fight that he WILL tap into the Speed Force, somehow...)
-Juliet Landau as the voice of the Scarlet Witch/Wanda Maximoff
I've used Landau as Wanda before, in my "Batman: The Brave and the Bold--Worlds' Finest" fancast, and I think she would have a really good take on the character (especially, like Witwer, with an Eastern European accent). Landau has a serious resume of geek cred, with three especially for DC Comics-based productions especially: Tala and Plastique on "Justice League: Unlimited", Labella in "Green Lantern: First Flight", and Drusa in "Green Lantern: The Animated Series".
-Kavan Smith as the voice of the Vision/"Victor Shade"
When I was first brainstorming this fancast, it was when I had just gotten into Eureka during its 4th season. Smith played Deputy Andy (an android) just on the cusp of grasping humanity, and I thought playing the Vision--an android who HAS actually acted more human than most--would be an interesting challenge for the actor. I imagine his voice being somewhere between Ron Rubin's (from "The Avengers: United They Stand") and Peter Jessop's (from "The Avengers: Earth's Mightiest Heroes") portrayals.
+Other JLA/Avengers Members
-Zachary Levi as the voice of the Flash/Barry Allen
Levi, in my opinion, is the best bet for playing one of comicdom's ultimate nice guys, the Silver Age Flash (or at least later, fully-developed depictions of the character), AND has the right voice for him.
-Adam Baldwin reprising the voice of Green Lantern/the Spectre/Hal Jordan
Baldwin voiced Jordan for a brief appearance in the Justice League: Unlimited episode "The Once and Future Thing, Part 2", and I thought he had a decent voice for the character. See this for example:
While this would be his second time voicing Jordan as Green Lantern, this would be the first time voicing Hal as the Spectre.
-Scott Bakula as the voice of the Atom/Ray Palmer
Well, this is the second time I've cast Bakula as a shrinking superhero, but this is also one I felt wouldn't be argued over.
-Kevin Conroy reprising the voice of the Phantom Stranger
I had to squeeze in everybody's favorite Bat-voice somehow! He really did a great job portraying the Stranger on "Batman: The Brave and the Bold", and I thought this would be a fitting cameo.
-Chris Edgerly as the voice of Hawkeye/Clint Barton
Edgerly's got a little bit of comic cred, having voiced (ironically, in relation to this fancast) Wally West in the Justice League: Heroes video game, Agent Haskett on Wolverine and the X-Men, and Gambit in the X-Men Origins: Wolverine video game. Playing someone as cocky as Hawkeye would be old hat for him.
-Gregg Berger as the voice of The Thing/Ben Grimm
Berger did a great Thing voice in the Marvel: Ultimate Alliance video game, so I brought him back for this lil' production.
Cap, while with Batman in the Batcave, uses a homing beacon to allow the Fantastic Four to transport a time-bike to his location. The Thing makes the delivery, and returns via a time-machine based on Dr. Doom's designs.
-Wally Wingert as the voice of Quasar/Wendell Vaughn
Wingert's portrayal of Henry Pym on "The Avengers: Earth's Mightiest Heroes" made him my favorite character on the show, and since Wingert's got a good comic character resume, he'd be fine for voicing Vaughn.
-Vanessa Marshall as the voice of She-Hulk/Jennifer Walters
Marshall has voiced a lot of comic characters in her career, and I really think she would nail She-Hulk instantly. Just read the line in the above pic with her voice in mind, as an example...
+Other major characters
-(Sir) Patrick Stewart as the voice of the Grandmaster
Stewart came immediately to mind when I was thinking of a voice for the Grandmaster--he's my choice if the character had been in a JLU-style Avengers show (or even ON JLU). And PLUS, it gives us someone from the Marvel movies (well, 'proto-' Marvel movies) in the cast.
-Anthony Daniels as the voice of Metron
Daniels was another face that immediately came to mind with this particular character--picture him playing a more serious C-3PO with a bit of HAL-9000, and you have a portrayal of the character right up there with Daniel Dae Kim's in "Justice League: Unlimited".
+Villain
-Leonard Nimoy as the voice of Krona
At first, I was going to cast John Noble, but with him voicing Brainiac in the upcoming "Superman: Unbound", I decided to go with another actor to play this mad seeker of knowledge. Nimoy did a great portrayal of Galvatron in 1986's Transformers: The Movie--well, just take a look:
Believe me, he would be PRIME for voicing Krona.
Additional Voices:
-Fred Tatasciore--Hulk, Beast, Solarr, Arkon, Loophole leader, Jarvis, Hawkman, Galactus
-April Stewart--Ms. Marvel, Black Canary, Artemis (Amazon), Oracle, Valkyrie, Huntress (Bertinelli), Hippolyta
-Khary Payton--Steel, Triathlon, Captain Marvel (Batson), Jack Of Hearts, Rick Jones, Black Panther
-Tara Strong--Zatanna, Poison Ivy, Dr. Light (Hoshi), Red Guardian (Balinsky), Prysm (Teen Titans),
-Masasa Moyo--Photon, Thundra, Lana Lang, Wonder Girl (Troy), Screaming Mimi, Nikki Gold, Crystal
-Yuri Lowenthal--Firestorm, Elongated Man, Wonder Man, Snapper Carr, Grim Reaper
-Greg Ellis--Namor, Captain Marvel (Mar-Vell), Sonar, Quraci cab driver,
-John DiMaggio--Shrapnel, Sandman, Hercules, Henry Peter Gyrich, Absorbing Man, Prometheus (DC Comics)
-Adam Wylie--Speedball, Beast Boy, Spider-Man, Nova (Rider), Zaran, Dreamslayer
-Corey Burton--Maxwell Lord, Nighthawk, King of the Royal Flush Gang, Felix Faust, Attuma, the Construct
-Dave Wittenberg--Green Arrow, Ace of the Royal Flush Gang, Batroc the Leaper, Red Tornado, Radioactive Man
And that, my fellow CBM'ers, is a wrap. I really hope you like this--heck, even make it front page-worthy! For taking the time to see this, I leave you now with the sounds of STAN BUSH: