I don't see a need to fully develope any new characters which could appear in X3. In the first movie, there was no background given to Magneto's Henchmen, and Jean and Cyclops were basically there to take up space on the screen. They could easily fit in new characters without adding backstory or fully developing them. It wouldn't be too far-fetched for a new X-Men or two to join the team in whatever interval of time exists between X2 and X3. Gambit could appear in the background, like Cyke and Jean in X1, and act like hired muscle. Beast could also easily be fit without much or really any explination due to the fact that the school is now missing one of its main professors and needs someone to take the place (two professors, if Storm is gone as well) I like the idea of filling in X3 with cameos and new characters, and not trying to limit limit the movie with two-plus hours devoted to established characters.