Kairi, to be honest. I said this before...but let's be real here, what do we know about Kairi? She's a resident of Radiant Garden, her world got destroyed, and now she's an islander. That's literally all we know about her. Should (On the slim chance that) Kairi EVER get her own game, we need to explore more of her character. We know NOTHING.
We don't know WHY she's a Princess of Heart, is it because of the light in her heart? Or is she, in reality, royalty? BBS & KH1, we see Kairi & her Grandmother in the castle. So...what if she's royalty? Then there's the Grandmother who seems to know more than the series lets on. People speculated her to be old Ava sooo...
I feel like, if we're to go in dept with Ven, and all this crap with the Age of Fairy Tales, then I feel that we know so little about Kairi. I've said this before, and I just think that such a thing shouldn't be avoided...like. Also, kinda like Princess Sofia, I'd welcome the idea of Kairi having connections and interactions with the other Disney Princesses, since she's a PoH like them. Like...the idea of calling to their power like D-Links.
We can also go into how Namine was made and how she differs from the other Nobodies. She & Kairi could be party members, and we can see what more could Namine do, given her powers of a Witch, and her power to twist around memories of those around Sora.
Y'know...the two Kairis have allot going for them, yet they're so underdeveloped, underused, and mistreated. I think...we know enough of the backgrounds of both Xion and definitely Aqua, but I feel we still know so little about Kairi & Namine. There's so much to explore on the both of them, and that alone warrants game focus, if anything.