Nomura never said it would take place between BBS and KH1 >.>
I hate when people be making stuff up...
All we know is that it will be a Secret Episode, not ending, with a playable character.
There's really nothing Aqua would be doing in the RoD, except for possibly running into Mickey. Don't know why that'd be playable though.
It could be XtA, but that'd be stupid. What would you do playing as him? A crappy experiment mini-game?
I don't like the idea of Braig. Who/What would he be fighting?
I like the idea of Sora, mainly because I miss him being in the spotlight, but I don't know what he'd be doing. Could be a sort of start for KH3D...
Vanitas is also a good idea. What was he doing on Destiny Island when Terra sent Ven there? This could answer that, but I don't know what he'd be fighting.