I really think they want an X4, but I dont think they can have it without Professor X, Jean, Cyclops, Magneto, Rogue and Mystique!
Cyclops could be replaced by Gambit on a powers-scale, as they are both longer-range attackers. Although Gambit's relationship with Rogue would be lost.
Magneto seems to have power left, so would just need to develop it further.
Rogue cant really be replaced, Kitty just doesnt have the power that Rogue has/had. Unless they make it so that Rogue didnt actually have the cure, and has found another way to control her powers.
Jean isnt really replaceable, but I'm not familiar with Psylocke, so she might be similar.
Prof. X wont be back as himself, but he would be Dr. McTaggart's patient, with his own original powers - maybe they could recreate his original body or something!?