The first decision Vaughn will have to make is if XMFC will be a prequel to the other three X-Men movies, or will it branch out in a new direction? Iceman and Angel were both original X-Men, but weren't portrayed that way in previous films.
If Vaughn and company want to deviate from the previous movies and stick to the comics, they can bring in Iceman and Angel, and get the original comic line-up. Storm is also a possibility, since she was a popular character from the first three films, and an older member of Xavier's school. Obviously, Xavier and Eric Lensherr will be around, but who else? Here's some ideas to consider while us True Believers hold our breaths in anticipation.
Polaris: As Magneto's daughter, Polaris wouldn't be hard to introduce, although in the comics, she is adopted by another family. She is portrayed strongly in Wolverine and the X-Men (The Animated Series).
Havok: Having Cyclops meet a long lost brother in this film might undercut plotlines for future movies, but it could provide a good ending. In X-Men: Evolution, Prof X does a lot of 'discovering' and finding Alex Summers could be a nice wrinkle. Then again, if Cyclops is just getting recruited in this one, there should be some space between the two.
Multiple Man: Turns up in The Last Stand, but who's to say he can't start out on the right foot and turn bad later on?
Sunfire and Thunderbird: Both brought on board with Wolverine and Storm in the legendary X-Men: Giant Size #1 comic. Since they didn't stick around in the comics for too long, they could be good fits.
Toad: Since he is already on Magneto's side in X-Men, maybe his origin is presented in this film.
Callisto: Leader of the Morlocks could join the team and leave for the sewers towards the end of the film. (Yeah, I know, probably won't happen)
Banshee and Moira MacTaggert: Moira, being an old friend of Xavier, could be brought in for moral support, and Banshee could tag along as her love interest.
Emma Frost: Since she appears in Wolverine with Scott Summers, perhaps she will stick around the X-Mansion.
In all likelihood, we'll have a nice simple team (hopefully the 5 originals, plus Xavier and Magneto, with maybe one or two others (mutants new to the big screen have been promised). I don't think we'll be seeing Wolverine, Shadowcat, Colossus, Nightcrawler, or any New Mutants, because rumors are flying that there will be a movie just for them.
All right, True Believers, what do you think?
