Personally, I'd much rather they gave us a Riku scenario if they give us another playable scenario. Xion is a main part in both Roxas's and Riku's story, and they gave us enough material in the trailer that would be all but impossible to show in a logical way not only in Roxas's scenario but in a Xion scenario too.
Xion and Riku on DI. Why should Roxas know? He shouldn't. Riku or Xion scenario possibility.
Riku vs Xion? Riku is supposed to win, thus giving it merit as having Xion be a boss battle for him.
And of course the scene between Riku and Mickey in the forest in Twilight Town. If it's playable and not say in the opening sequence, neither Xion nor Roxas seem likely to even be around for the most part.
Xion's story is important, but because it is important you won't get to play as her, I think. That's because her importance is in how she affects Roxas and Riku, not in her getting through her own story. Sort of like how Riku was in KH1 - Riku had his own story and he went by it, but for the most part you didn't even need to see it, only how it affected Sora, being the main protagonist.
Not ruling out a Xion scenario, I just think that a Riku one is more likely, especially since Riku's story is one of the four stories Nomura said he wanted to tell, being -
Roxas's time in the Org (Days, obvious scenario),
The time of the King's Absence (Coded, a bit of Days)
Xehanort's lost Memories (BBS),
And Riku's time away (Coded, and hopefully Days).
I doubt Riku'd get his own game anytime soon, so him being a playable scenario is likely.