1. Cable
Cable is almost the perfect X-Men for the sequels and could easily be introduced in the Days Of Future Past storyline considering he is from the future anyway. However, I would introduce this character in the sequel, however save him for a big arc in X-Men 4 and 5. Ever since I heard those rumors I have always thought that X-Men 4 and 5 would likely be telling the stories of "Messiah Complex" and "Second Coming"- two of the most recent and exciting X-Men books. In fact, the stories are made for film, and tell the story of Cable time traveling to save a young mutant messiah with "phoenix" level powers from those who want her dead. Think a cross-between Terminator and Looper (in fact, it is very similar to Looper) then you have the general idea. For those that don't know this very popular character, he is the son of Cyclops and Madelyn Pryor (who happens to be a clone of Jean Grey). Though he was born in the present, he was sent to the future and raised there. When he does return to see his father Cyclops, he is a bad-ass futuristic warrior with gray hair who is older than his Dad. He has similar powers to his mother (telekinesis and telepathy) and can also manipulate technology. He is great at hand to hand combat and is skilled with a gun, regularly carrying large futuristic super weapons. There are rumors of this character having his own film, however he would be much better introduced at least in the X-Men films and then span off if all that necessary.
WHO SHOULD PLAY HIM: You would need both an older actor and a very physically fit actor to play this bad-ass role. If you could convince the actor,
Harrison Ford would be beyond epic as this character... although perhaps he is slightly too old. Even still, imagine how cool that casting would be. If he refuses I have a few more potential options. How about
Richard Gere or
Michael Douglass in comeback roles, or maybe even
Ray Liotta? If all else fails,
Bruce Willis probably would do it and no one would really object to this. These are my options. Instead of telling me why I am so wrong in the comments sections, I'd much prefer to alternatively hear who you'd cast instead...