Inhumans:
A very important and closely-tied extension of the Fantastic Four, for which the movie is undergoing rebooting itself, the Inhumans would be a great story element to a new FF film, and therefore already be well set-up for their own spin-off. Unfortunately due to licensing issues, this would shut the door on an opportunity for the Inhumans to meet up with aforementioned cosmic characters(Nova, Guardians of the Galaxy)....and that's an extended universe I'd love to see.
Thunderbolts:
Imagine over the next several Marvel films encountering on different levels the villains of Baron Zemo, Moonstone, Ghost(for whom I read a comment by one of you saying he'd make the best Iron Man 3 villain, and I agree)....then all of them becoming a "superhero" team in disguise. After a couple of Avengers movies, how awesome would it be for the Avengers to be the "villains" in a well-written Thunderbolts movie that could take an alternate look at the Marvel U. Ant-Man would be a great addition, as he was in the comics as the T-Bolt's anti-anti-hero with less than evil intentions. Once again, licensing rights tie up potential greatness: Norman Osborn as leader/crazy dick and Venom as a man-eating member, due to their characters belonging with Spider-Man, and Bullseye is likewise stuck to Daredevil.
Luke Cage/Iron Fist:
I have doubts about how successful as mainstream properties these would be as two separate films, but if Marvel were to embrace these guys' long history together, they could make a very interesting superhero/buddy flick with two unique characters. Play off of their glaring differences in a smart, not hokey way, but also focus on why they're friends, co-workers, and why they trust each other in the pursuit criminal assbeating. Then throw them into Avengers 2 or 3. Done.
Ms. Marvel:
A great opportunity for Marvel to pick up some female fans. And the guys will like it, assuming its well-made. I just spent way too long searching through pictures of her for one to use. (You're welcome for choosing the one I did.) She's built into the overall universe as a SHIELD agent. Her origin would be easy to incorporate and unique as well. Once again, give us a movie with her, then throw her into the mix for an Avengers sequel.
The Flash:
One of the few easily-identifiable comic book superheroes yet to be made into a film, the Flash has a lot of potential if filmmakers choose wisely from his long history. It would be an awesomely visual movie, and though I'm not a huge fan of 3-D, this one done properly could be the best 3-D film ever. And humor has always been a great aspect to the Flash, so get Ryan Reynolds to play him too. I kid...but maybe Chris Pine would be a good pick. This movie is only a matter of time, and I'm honestly surprised it hasn't gotten past "in development" phase, though IMDB has it listed for 2013.
Black Panther:
I'm pretty sure Wakanda was one of the spots on the map in Iron Man 2, so Marvel is already planning this guy's debut. One of several excellent opportunities to spotlight a badass black character, Black Panther would have cool technology, fight scenes, and the combination of naturally-beautiful African landscapes and the highly-advanced society in which he rules. Throw in a threat that could affect America as well, and SHIELD and certain Avengers can cameo. Follow successful formula: 1. Make High-quality origin feature. 2. Add him to the next Avengers flick, and Bob's your uncle. Thanks @intruder for the pic.
Preacher:
This one's tricky. I don't want to say it can't be done, but this would be as close to "unfilmable" as Watchmen's Alan Moore said his property was. Now since I like the Watchmen movie despite its faults, I'll support the proper handling of Preacher. With such a finite story compared to most comics, and with each little aspect of story and character development being, in my opinion, essential to the overall experience, maybe it would work better as a TV series, similar in quality to The Walking Dead...maybe on HBO? Or a few well-made hard-R movies? The pic I found is disturbing and delightful.
Who else you got?