Kairi should be the one to plunge willingly into the realm of darkness. This seems like a task destined for her. It should be ok since her heart is pure light the darkness shouldn't be a threat and she would actually thrive in that world pwning the darkness and help Aqua escape with her light powers or something like she helped Sora and Riku at end of KH2.
Once Aqua is freed Sora journeys to CO with her and finds the Chamber of Awakening to return Ven's heart back to him (not sure how it will be given back though)
Then Riku should be the one to defeat the Terranort Xehanort and free Terra and expose that oldnort noob out of hiding.
Once Terra is freed then Sora Riku Kairi Aqua Ven Terra Lea (Saix too I hope) Donald Goofy and Mickey all do a final showdown and thrash the living hell out of all the norts and send them to oblivion.
While I love the idea of Kairi being playable, I wouldn't want anyone to take Donald and Goofy's place.
If, IF I had to make a prediction about playable characters, I'd think that Sora, Donald, and Goofy do their thing and team up with anyone willing as per usual.
Riku's journey would be a solo adventure using something similar to D-links (drive forms?) instead of party members, except for Mickey at key points.
Kairi, in the meanwhile, could go any which way; I'd guess that she has no permanent party members but cycles through other Disney party members as well as major character such as the keybladers, with a theme of unification to loosely parallel Sora's theme of connection and Riku's theme of inner-strength.
Honestly at this point SDG will only be together in the disney worlds. There's too much going on in the storyline where they aren't important.
And what's keeping them from being important?
What does Donald or Goofy have to do with rescuing Ansem/Aqua or saving Ven???
Mickey has to with Sora to save Aqua. Sora has to go with Aqua to save Ven. Now of course D/G should have some part in the finale battle. But i just don't seem them of any importance to most of the main plots KH3 has to handle.
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