First I'd like to welcome you to the forums, I hope you have a good time.
Honestly if its really called deep dive then your theory is probably right, but don't you think it would be better if it was Riku? After all he's one of the main characters, and moving Kingdom Hearts to a whole new level (Ps3) would be much more worth it if it was about a character better than mickey, after all Riku was also in the realm of darkness and not only mickey.
Don't get me wrong, am glad there are others like mickey who can wield a keyblade, its just that moving Kingdom hearts to the Ps3 would be cooler, or lets just say more attractive to people if they were to play with a character lets just say more worth playing with.