It just doesn't make sense that they would put the words "Birth by Sleep V.2" at the end of the BBSFM secret ending and then not follow up and tell the story. Nomura is quoted as saying, "There is a story that exists [for Birth by Sleep V.2]." If he's telling the truth, then it's inevitable that the sequel story to BBS gets told, since it must be so vital to the Xehanort saga as a whole. But will it get its own game?
It continues to less and less likely that this will be the case. In his interviews, Nomura seems to hint that there simply is not enough time or manpower to make the game at this time, while sticking with his 1 game per year promise. This makes me think that instead, bits and pieces of BBS v2's story will be placed into KH3. It could be interesting if every so often in KH3, the main storyline with Sora and company paused for a bit to show lengthy flashbacks from the BBS v2 story.
Or it could even go a step further, and have the player take control of a BBS character for an episode or so (similar to how Roxas was controllable in the beginning of KH2). Either Aqua in the realm of darkness, Terra fighting for control of his body inside of Apprentice Xehanort, or even Apprentice Xehanort himself could be playable for a short time. It would make the overall story clearer for the gamer, so when Terra/Aqua/Eraqus appear again, everybody is not confused about what they have been going through.
It seems like an idea that Square would think of. The storyline of KHII was confusing while playing through it, showing random flashbacks of Roxas before we even knew who he was or how he was relevant in the story. But in the end, it all came together and made perfect sense. Plus that seems to be a real fad in media these days--starting with a random clip that makes no sense only to have it clarified later on.
Who knows what Nomura has in store regarding the BBS v2 story. He may have yet another curveball us his sleeve, or he may just simply want to clarify some things and tie up loose ends. I know I still have plenty of questions--like why is Apprentice Xehanort look different (face, hair) than Terranort does at the end of BBS? And what is the status of Terra/Eraqus? Or did Aqua ever encounter Riku, Mickey, or any other characters while in the Realm of Darnkess?
These and many other questions need to be answered, but probably won't be in its own game. So its logical to believe that the mysteries that still remain from the Birth by Sleep saga will be answered in the numbered title.