Never say never, but from the sounds of everything I've been reading this week the spin-off films will be the "sequels". So here are the possible films I've heard about thus far:
Wolverine- Pretty much a sure thing, and most likely the next X-men related film that will hit theaters, possibly around 2008.
Magneto-Can tell his life story without coming up with a convoluted, soap opera style way of bringing back the character in the continuity of the first 3 films. My guess is that this is the second most likely to happen.
Emma Frost-She never appeared in the X-Men films, but I read today that a film based on her character is possibly in the works.
Xavier's school- Another film I only just heard the first rumors of today. It would focus on the younger mutants who are students at the school. It would be more about their day to day lives than the greater world saving missions of the previous films. I'm going to guess that it won't happen, but it could be fun if it's handled correctly. Would also be vastly cheaper than any of the other films.