Will it allow you to play as multiple characters (Sora, Riku, Aqua, Ventus, Terra...) or will it be a standalone adventure for Sora?
BBS did a great job building up these characters and making them fun to play as; it would be a shame for it to be their final outing
While I sadly doubt we will get to choose our party leader (in that case I would ditch Sora immediately and run around with Riku, Aqua or Ven), I could imagine we will get maybe somewhat of a party system either in KH 3 or in the next arc.
I would surely hope that we'll get to play as TAV again someday, playing as them was way more fun and satisfying than KH 1 and 2 together for myself.
I just loved Aqua's total versatility in magic, her neat barrier and cartwheel.
I also adored Ven's unique backhanded style (which will hopefully
remain unique), his enormous speed + wind and light magic as well as his personal command styles.
Even Terra, while somewhat slow, had his moments when you could crush an big enemy with just two or three hits because of his vast power...*ggg*. Give him Ven's D-Link and even he gets somewhat faster.
I kind of hope so, even if this is Sora's story. None of this wouldn't have happened if it weren't for them. It'd be cool to be able to play as them again, or have them as allies in battle. I get the feeling we won't even see them until Sora rescues them or whatever the hell.
It mainly being "Sora's story" doesn't mean that he has
always to be the spotliight hogging squad and be the only playable character. It was exactly BBS which showed that a decent KH experience is possible without Sora himself directly involved.
Nonetheless, in a main (meaning numbered) game I can see them rather as allies/party members rather than actually playable.
As far as the trailers of DDD showed we will at least see them, but only confined to cutscenes. Pretty much the same as it was with Roxas after Days and the KH2 Prologue, only cutscenes and in actual gameplay we were stuck with Sora, Donald and Goofy.
Honestly I'm leaning towards them becoming secondary characters or party members rather than full playable characters again.
I'd be nice for a KH game to have FF-like parties where you can choose whoever you want to have in your party and be your main (while the story still surrounds the main character)
As much as I dislike that, I have to agree at least for the main numbered titles, except if we really get something like it was suggested several times already, a full fledged party system to which new characters are added as the story progresses.