I would hope that they could get all the essential X-Men that "survived" X3 for an amazing future sequence. I'm talking all-out displays of power, a final high-stakes mission, and ultimately... their deaths.
If I had my choice, I would bring back Logan, Storm, Rogue, Kitty, and Colossus (and maybe Nightcrawler if the budget can include him). I would establish that Beast, Angel, and Iceman had perished. And of course, Magneto would be fighting alongside them to show that all mutants are fighting together for survival.
The franchise has always been blessed with more talent than they've known what to do with. Here are the "future" X-ladies as they appear now showing off the look and attitude that they were never able to display in the movies.
Kitty is smarter than you'll ever be, Storm is the goddess, and Rogue is the Southern belle/femme fatale.