I am just curious as I thought there would be a secret 'playable' ending on final mix? BbS volume 2 would be a new kh title game? Well, I didn't expect that o.o but anyway, there's no way that BbS vol. 2 = Kh3.. since KH3 comes after kingdom hearts 3D which isn't even release yet. My thought is that like most people said, this 'side-story' just fill the gap between series. perhaps, nomura wanted for people to understand (might confuse some?) the story as a whole.
I've read somewhere that these series from BbS all the way up to KH3 are just a part of much bigger adventure and Nomura even entitled it as 'Xehanort's Chronicles' There might be some more new enemies to come. Who knows, Sora's adventure may end and a new boy/guy would come up holding a keyblade.
I am really hoping that KH series would end up like FF series although KH actually goes on with a sequel rather than 'start-end' kind of series. I would really love that.