They could easily stretch a full game with just Aqua in RoD, but I think there will be multiple scenarios (if BBSv2 is actually a game, which is most likely). First off, they wouldn't call it Birth by Sleep Volume II if it didn't at least share
some similarities with the first. (Of course, the name can change, but let's take it that it won't.) I just think that it'll probably follow the same kind of path as BBS by having three scenarios, just to make it feel more like a continuation of that game. ...If that makes sense.
Also, I think more scenarios are likely because, as Nomura said...
(Source:
Director's Secret Report XIII)
The last two stories have already been told in Days and BBS, respectively. Both of those were considered important enough to warrant their own game. Placed at equal importance, Nomura also mentions Riku and Mickey. As of yet, these stories have not been told. We know that there are only two games before KH3; DDD and the 'Mystery Title' (most probably BBSv2). I can't see them squeezing the two stories in DDD (maybe some of Riku's), so that just leaves BBSv2.