Originally Posted by Angamb
to me Singer came back to Fox with the FC script because he wanted to have at least one final movie with his cast, since he didnt had the chance back in 2006, but since Fox wanted other thing around 2009, he came with the idea of the prequel set in the 60's, to develop some of the characters he worked with and to connect it later to DOFP and finally do X4 right after.
Seems like a really long plan, and probably some fans wont think this way, but if this is really the case, yeah, its a smart plan.
FC expanded the x-men universe, and now Fox has the chance to keep expanding it with any of the two casts or both.
It doesn't make sense to make a direct sequel to X3 now. Specially when the events of Days of Future Past will change the events that transpired in the original trilogy. This movie represents the closest we'll get to an "X4" in that it reunites the cast of X3 in a supporting nature.
It's not even a question of "why?" would the studio want to make a sequel to X3 but rather "How?" could they do it now that the timeline where X3 existed has been altered by the events of Days of Future Past.