I totally see where you're coming from with that, because it's largely true of her as a character; but up until now, that's simply been her role. She's Sora's (and, I guess, Riku's) "light" and her journey up until now has been circumscribed to theirs. I don't approve of her writing in the least (but, I reiterate, poor writing is not something that she alone is victim to across the games), and that's why I expect and hope for a shift in dynamics in the upcoming game. Whether KH3 is the end of the line or not, it can't be denied that is the end of an era: much of the audience for KH is cashing in or potentially cashing in with this installment, and Nomura and Square had better know that. Whether they can revitalize their fanbase with another saga remains to be seen, but this is essentially the last chance for the KH team to give every character the resolution they deserve without cheating the players, and if Kairi's fate is simply to be snuffed yet again, how is that any better than taking a chance and giving her a strong role to fill? Whatever writing her character has been subjected to in the past shouldn't preclude her involvement in the finale.
I suppose I simply don't see how, barring some impossible scenario where Kairi becomes the main character and Sora dies and Riku has her babies, she could disturb the progress of the storyline as is. She would just be another Keyblade wielder. At this point that's hardly even an accomplishment. (Joking. Mostly joking.)
Well, honestly, Sora slept a year between CoM and KH2. I don't imagine it would strain disbelief for Kairi to be training for some time and join up with the party in the middle of the game, maybe jumping in during some terrifically dramatic boss fight. It would be classic Square Enix, actually. Perhaps Kairi's training could even function as a kind of tutorial, similar to how we started out as Roxas in KHII. After all, they have to fit the boring instructional section of the game in somewhere, and Sora and Riku have little left to learn. Moreover, Kairi has already wielded a Keyblade, prior to Lea, and by all rights she should have been able to summon a Keyblade already given her encounter with Aqua in BBS. It's just that the writers hadn't thought of it yet.
Again, I'm not saying I know what's going to happen, I'm just saying the evidence is invariably in favor of her participation as Keyblade wielder in KH3. At this point it would make little sense for the game not to incorporate her in that capacity.
My problem with that is Kairi expressed explicity in KHII that she was no longer in the business of being left behind and held hostage/prisoner/whatever. (I know, I know, then the writing promptly expelled her from the final boss fight and left her on the Islands while Sora and Riku went off to do their thing, but the former was simply a matter of convenience and the latter was just piss poor characterization). My point is, to instruct her to remain far from the battlefield when everyone she cares about is risking their lives is not only classically pointless (so she's safer by herself far away from everyone else who can fight and help her in the event that someone tries to kidnap her again? yeah that worked really well the first two times), it removes agency from her character to take control over her own fate. Every other character, including her Nobody and pseudo-clone, has had the opportunity to determine their future for themselves: whether by fighting for it, or giving it up for the greater good. Kairi is the only one who is constantly subjected to the will of others, without fail and without having the chance to freaking punch a Saix. I'd like to see that trend smashed good and hard with KH3.