I'm thinking this might be another multiple POV single-quester like Birth By Sleep was, personally. The premise, IIRC, is Sora and Riku taking their Mark of Mastery examination, but I don't think that in and of itself is going to be enough to be all that's going on. I'm betting that Sora and Riku get to Yen Sid to begin formal training as keyblade masters, but something probably separates them and needs to be dealt with before either can truly claim the title. I'm not so sure it's likely, but I'm also hoping Kairi's own little quest starts up as a third POV. Might be a little early for Aqua to escape the Realm of Darkness, but if she does, I think she's going to start training Kairi.
Mysterious Figure might play a big role in DDD, and I'm betting on him turning out to be Eraqus's amnesiac Nobody. Expecting some young Xehanort/Eraqus/Yen Sid backstory, too.