Spoiler

In my opinion, KHIII kairi sucked. 

It's a silly idea, but I really would've loved it if things took a big turn and kairi fight vanitas. Then suddenly out of nowhere, the X-blade is forged. Yup, turns out kairi was as strong as vanitas was! Then we would get to play as her and beat the crap out of vanitas. Then she uses the x-blade to close kingdom hearts. Would've been nice character development for her in my opinion.

 

now that you bring it up, what if she fights vanitas and than sees his face which really is hard for her to handle even though she knows its not really sora.

Maybe she sees his face and ends up turning him to the light.

Spoiler

 

Spoiler

What if when Kairi "died" she actually went back in time and lost all of her memories. She had the ability to weild the keyblade so she was taken under the wing of MoM. She then became known as Ava. I believe that Ava is the subject X that is mentioned in the seceret reports.