agreed with much of what's been posted above. I for one would love a scenario where Sora once again is out of commission or separated from Donald and the Goofster, and they end up with Kairi. It doesn't even need to be a playable situation, just a few scenes popping in with those three; they've got a lot of common ground! No better way to strengthen Kairi's relationship with Sora than by having her connect with the people closest to him.
That and she was basically with Sora for the duration of his KH1 adventure, so she knows them better than you think she might.
I'd like for KH3 to once again keep Sora, Kairi, and Riku split up for most of the game... Make them go through stuff and then have another kinetic exciting reunion when shit's getting exciting in the story. Let Sora and Riku be surprised and relieved at Kairi's budding independence. They're so used to protecting her that I'm sure letting her go free is both worrysome and uncomfortable. Don't just lock her in a training chamber, have her go out and do some important stuff.