To be perfectly frank, we all know there is that bit of sexual tension between them, but god no. I'm sorry to disagree here, I really am. But they are meant to be friends, yes a little more than that, but they aren't supposed to be romancing with one another, especially with Riku on the sidelines. It could be argued however that this would put all the Sora x Riku shipping to rest, and that'd be quite something. I honestly don't think that the relationship between Sora and Kairi, that being that they romance each other, would coincide with the thematic approach that Nomura has in mind. As much as we hate to regret it, the game does have a younger audience, so the romance to them won't be understood by them as much. Anyway, that is just my opinion.
It could be pulled off though...