It would be interesting to see Forge appear. He was the tech genius for the X teams. As for future X-men related stuff, I don't have high hopes for the Gambit movie. Been hearing too many things, and just got this really bad feeling. When they said it would be a cross between a heist movie and magic mike, my head hit the desktop.
Stephen Lang would be interesting for Cable. Someone dropped Dolph Lundgren's name for the role. Or, even Sean Bean for Cable. Or, if they are going with a slightly younger Cable, Karl Urban. He has a decent track record of action movies, and he played Judge Dredd in the reboot movie, Dredd.
I would love to see them branch out and try to tackle some of the other X teams like Excalibur, Exiles, X-factor (with Havok, Polaris, etc), New Mutants, X-Force.