Unfortunately I don't see us being to play -- or take control rather -- of all seven of the Guardians of Light. I think that just puts a lot of extra work for Square: having different combat styles, animation, etc. I don't know, it just sounds like a workload.
However, I do love the idea of a quest arc in the game. Taking on more tasks from NPCs to interact more with the worlds would be a complete change, since usually (I've found) the series can be linear. The worlds are all empty except for Sora & friends plus a few main characters of whatever world their in. From the trailers it looks like it's going to stay this way but eh, I can hope.