Haley Ramm as Barbara Gordon/Batgirl
As Arrow dives deeper into the Batman mythos and rumors of Nightwing appearing, it isn't crazy to think we may get Batgirl sometime in the future. Haley Ramm is a good upcoming actress. She's played Jean Grey, been on CSI, The Mentalist and did a voice in LA Noire.
Max Thierot as Lonnie Machine/Anarky
With Arrow's stories mostly involving politics and government then Anarky could be the perfect character to introduce. He possesses a genius-level intellect, and is particularly skilled at computer hacking. Anarky is also an adept martial artist, brilliant inventor, gifted strategist, and tireless crusader against authoritarianism.
Nina Dobrev as Atomica
A.R.G.U.S is a thing on Arrow and Atomica could definitely lead to the creation of a full-fledged DC TV universe. She has ties to Doctor Light and Superman so that could definitely lead to bigger things.
James Wolk as Onomatopoeia
This could be a really interesting character to bring into the Arrowverse. This could be our departure from the 'hired gun' and the 'vengeful' villains and get into the criminally insane side of things.
Ron Perlman as Solomon Grundy
Remember those two seconds Solomon Grundy was on Arrow? Look it up it was supposed to be him. Anyway, that was a bad way for a good villain like Grundy to go out. So with all the Mirakuru madness out of the way it would be cool for Grundy to just make an epic comeback and pose a true threat to either Arrow or Flash. It's also no secret Perlman likes comic books or at least likes doing comic book related things.
Julia Ling as Blood Rose
With the next season of Oliver's past taking place in Hong Kong this will give them a chance to introduce alot of new characters including Blood Rose and Katana, the latter already confirmed to be in the show. Since Arrow is grounded in reality I think they'll get rid of the 'android' part of her character but still have her be a gun-toting baddie of Oliver's past.
Kevin Michael Richardson as the voice of Gorilla Grodd
If you saw the leaked Flash pilot (if you didn't then SPOILERS) then you probably noticed the Gorilla Grodd Easter egg. If they were ever to bring this simian villain to the small screen then the actor mostly known for his deep voice and voice work should help bring him to life.
Cameron Boyce as Garfield Logan/Changeling
Flash could do a really interesting take on this character. With his animated counterparts being more comedic this could be the opportunity for the character to get some serious overtones. First step is not going by 'Beast Boy' though. Either way they chose to the character I'm sure Boyce could pull it off easily.
Yvonne Strahovski as Jaina Hudson/White Rabbit
An interesting character considering she can split into two different forms of herself. A master escape artist with ties to Bruce Wayne, she could be a formidable villain on either show.
Matt Bomer as Bruce Wayne/Batman
Arrow and Flash have name-dropped the Dark Knight and since it won't be a part of the DCCU, alot of fans are wondering if we'll get our own JL on television. It may be far-fetched so I just say since there's obviously a Bruce Wayne in the Arrowverse have Matt Bomer play the millionaire. Only if he's playing Bruce and not the actual suited up hero it will give fans some confirmation on where Arrow stands within the timeline of the Batman mythos. Also, having Bomer in full Batsuit is a hell of a way to end a season off.
James Roday as Ted Kord
Arrow or Flash could do a good take on Blue Beetle, a character I hope pops up soon since many people forget about him. Ted Kord is the mastermind behind the now infamous Blue Beetle that has been spotted around Starling City. Kord has retired himself from the Blue Beetle mantle and has passed it down to young Jaime Reyes.
Avan Jogia as Jaime Reyes/Blue Beetle
A talented young actor that is looking for the next big thing to do after his swan song with the show Victorious. Avan could definitely portray a Young Justice-esque version of Blue Beetle.
Bryan Cranston as Lex Luthor
Cranston's primary medium is television so it's not outlandish to think he would do a television show again. Everyone pretty much wanted Cranston for Luthor or Gordon in Batman v. Superman. Some even speculated it was true. In the Arrowverse we could get the Luthor we wanted in the first place.
Mila Kunis as Harley Quinn
I doubt the actress would appear on television or find a way to work around her upcoming films to appear on the show but if Harley Quinn is ever brought to life on the big or small screen it should be none other than Mila Kunis. Her performance as the Wicked Witch in Oz the Great and Powerful is as close to Harley Quinn as possible.
Joseph Gilgun as Dr. Jonathan Crane/Scarecrow
A talented actor that could portray the man known as the Master Of Fear. This villain would most likely appear on Flash considering Arrow has just recently dealt with a similar villain in Brother Blood.
Vanessa Hudgens as Jade Nguyen/Cheshire
Hudgens really fits the bill if Cheshire is to appear on Arrow anytime. She's multi-racial. She's a decent actress and she matches Colton Haynes (Arsenal) in age being only one year apart (I think). Hudgens needs her big break because she has proved she can bring the sex appeal and badassery (Journey 2 The Mysterious Island).