Kingdom Hearts, as stated by an interview with Nomura, is Sora's story. Any game with Kingdom Hearts in it has to pertain to Sora in some way/shape/form. If you look at everything that is coming out, there is a strong link to Sora through it, be it a game explaining the oddities of Roxas, or a game explaining why Sora was "Ten years too young" to weild a keyblade. It all fits pretty well.
Plus, it has been stated that Xion will serve as a reason for Roxas' deflection from the Organization. So, yes the game will have a good bit to deal with her, but only because she is oen of the reasons Roxas left, and possibly the reason that he even was caught by Ansem the Wise and the events in KH2 could happen.
I will be honest and say that the idea for spin offs of the series that do not explain questions left unanswered by the first three games would, to me, seem a bad idea on Square's part. That doesn't mean I don't think it can't happen, it just means I'd rather not see it.