All valid points about Kairi's lack of character development post-KH1 aside, I think there's enough evidence in all the games that people have already listed here that all suggest that Sora and Kairi are destined to end up together. Sure, the series emphasizes friendship overall but the special bond between those two, I feel, largely eclipses the bond between any other characters.
I also recall reading an interview Nomura did shortly after KH3 was unveiled E3 2013 that said that in some ways, KH3 would "grow up" in the same sense that the players who have been with the series since the beginning have. If this goal still holds true, I hope one of those ways mentioned ends up being through Sora and Kairi's relationship. I think there are plenty of ways to do this without it coming off as "cliche" or to diminish any other part of the game.
I really hope you're right, DoctahFaz. And not just because I do ship them, or whatever (and tbh, I care much more about seeing their friendship developed--heck, Kairi's friendships with
anyone--more than anything else these days), but I need this.
Mainly because if there were a relationship upgrade, or if they just
talked about their feelings instead of skirting around the issue, it would finally stop them being so awkward around each other, which...
Is nice and cute for their age group and situation; I get it, I do (as I also went through the, "OMG, I love them, but they're my best friend and I better not wreck the relationship by saying anything" stage), but I really want to see them natural and comfortable with each other: the kinds of best friends they're
supposed to be. And I don't think we're going to see that at
all until the elephant in the room is finally addressed.
Also, KHIII has a lot of ways that Sora and Kairi
could bond, I think. Kairi's a huge target now, so more likely than not Sora's going to be concerned about that. And who knows? He's all ready expressed guilt about what's happened to her through him before, so maybe now he'll finally try to keep good on his promises to protect her (or at least a bit more seriously than he has before)?
Or the complete opposite of that could happen--where Sora just feels awful about everything that's happened to Kairi, and doesn't want it to happen again--but Kairi sets him straight: She lets him know that she doesn't blame him for anything, and that she can handle herself just fine.
There's also the possibility for some friction between the two, because I imagine Sora is going to have to deal with his darkness in this game, and Kairi has none. So that could maybe lead to a wedge between them that needs to be addressed. (Maybe something along the lines of Kairi trying to get Sora to open up about it, so that everyone can understand and try to help him some, to which he'll reply she literally
can't understand it.)
Also, things like Sora
maybe not wanting her to fight could also add some drama.
If KHIII delivers, we should hopefully have a lot more Sora and Kairi development in the game, which I think is good and very much needed.
I'm especially hoping that we'll see some moments where they're not seeing eye-to-eye with each other, or are maybe angry with each other some, because so far that
hasn't happened in canon.
So, yeah: if KHIII develops Sora and Kairi's friendship in such a way, a relationship for them could probably work easily off of that in the future (and not seem as underdeveloped and out of left field as it kind of does now).
...IDK. These are just my two-cents on it all.