Continuity soup. But this is a very fine soup!
coffeeem called it: best tragic buddy movie evar.
They could just leave this. It doesn't need a sequel. But of course, they won't. I hope they don't try to rethread it up to the last batch of X-Men movies, though. I liked the first one just fine, and the second one kicked ass. But they don't fit this one, and I hope the powers that be don't try to force it.
I do think this movie was the best thing every done with the character Sebastian Shaw, and not just because it's Kevin Bacon playing him. Though Kevin Bacon is awesome, of course. But in the comics it always comes off like Shaw was an interesting idea for a character but they never know what to do with him. Everyone around him does stuff, and he's just sort of there being pretentiously menacing or something.
So what can they do with a sequel? I don't know. It's pretty much guaranteed, the way the movie industry works, that there will be one. I hope they think it through really well, and stick with what they created with this movie rather than trying to get it to fit any other X-Men continuity.