This is the next Kingdom Hearts game not a spinoff people. You see that the story starts on Destiny Islands with the current timeline Riku and Sora who somehow end up in their younger forms. Days and Re:Coded expanded on already known parts of the story however KH 3D and BBS are new stories. BBS is the prequel to KH and 3D will connect 2 and 3 much like Chain of Memories connected the first two KH games.
Instead of worrying about whether or not it's a spinoff how about looking at all the gameplay elements they're adding in to this game. Riku is playable, the battle system is much quicker than the original games, and it looks like they're going with a darker tone with this game. Apparently the game's tagline is "Darkness becomes light, light falls to darkness." which sounds like Riku and Sora might trade roles eventually. I dunno this seems more interesting than I was expecting.
