His name was in it, that doesn't count. It's like saying T.H. White was in it because his name was on the book that Magneto was reading.
But, anyway, if I were a filmmaker, I would stay away from solo spinoff films. It's just a way to try and make more money. Thing is, it hardly ever works. Not only does it not work on the silver screen, it doesn't work on the television, either.
A Wolverine movie would probably make the most money, but it would no way in hell make the same amount of money that a X-Team film could earn.
So, if I was making a spinoff film, I would most likely make a prequel of sorts, revealing to the audience and fans the X-Men's first adventure against Magneto or some new villain. The roster would include Cyclops, in an effort to give him more of a leader-type role. Jean would also have a large role in the film (pre-Phoenix, of course). With Storm and Beast also on the team, there is always the oppurtunity to bring in a character that has yet to be introduced. Banshee, perhaps? Havok? Maybe even Forge?
I'd have set in a X-Men Evolution-type storyline, showing the X-Men in hiding at first and then be revealed to the general public.
But, what the heck, that's my two cents.
A prequel or any other X-Team combo (including some of the older castmates) would generate the most money.