They could also as easily make Mickey playable, who hasn't been playable yet outside of a few battles in KH 2 only under certain circumstances or expand on the assets they already have for Aqua herself from 0.2 (as I mentioned already) and tell the story from a flipped point of view with occasional cutscenes of Riku and Mickey interwoven with Aqua's gameplay and a central objective being them meeting up eventually.
The possibilities there are pretty vast actually and Riku already was playable more than any other character except Sora himself.
I get the sentiment when it comes specifically to Lea pretty easily, yet these tendencies to insist on that Kairi must be playable across the net aren't limited to when it comes to him alone.
It almost comes over as some 3/4year olds in the defiant phase throwing a tantrum over this whenever the topic of additional playable characters comes up despite actual playability not being any guaranteed ticket for actual better involvement and characterisation.
That's why I stressed I am making a general observation on the topic, I wasn't pointing fingers at anyone in particular.
Forcing Kairi down people's throats won't really work I'm afraid and may on the contrary hurt her standing even more in the long run and if not her as the character itself, certainly the reputation of her fans will be.
The main issue with Kairi is also that unlike nearly all the other major original characters, she has close to no prior setup for an arc that could be continued going into the final game of an entire saga.
The only one I could imagine having a similar predicament is Naminé, but she did at least gain some small premise by apparently being able to somehow contact the Lingering Will/Terra and aiding in somehow bringing everyone together at last, but what has Kairi?
With all the stuff already going on on top also constructing a whole arc for Kairi on her own confined to KH III's boundaries is certainly not impossible, but in order to be pulled off truly well would require more focus solely on her that I don't think KH III can reasonably provide without harming other already existing arcs needing to be addressed.
It would have been a better solution if DDD actually took care of the setup of an arc for Kairi to follow up upon in KH III, and that's where I am in full agreement when it comes to Lea being pushed to the forefront being an error, yet that's the status quo one has to deal with.
I don’t have any issue with Mickey being playable other than they may still want to focus on the OG characters. He’s super OP in KH2, but at this point, I think Riku is almost his equal as a Master so it wouldn’t feel too much like they’re unequally yoked. My personal preference would be to play through the Realm of Darkness scenario as Riku with maybe some segments as Mickey like if Riku gets KO’d, but once we beat it, we can revisit explored areas (maybe including ones from .2) and play as Riku, Mickey, or Aqua at will.
The main reason I wouldn’t want the RoD gameplay to focus on Aqua is because that seemed like the point of .2, which was a direct lead in to KH3 and specifically set up for Riku and Mickey’s plotline. I don’t think there is much story for her now other than waiting for Riku and Mickey to get to her. They’ll likely find her shortly after she meets Ansem the Wise, syncing up the two plots in a way that leaves little room for additional stuff from Aqua’s perspective.
For Kairi and Lea, I totally get why the lack of set up for Kairi’s playability might be frustrating, but pretty much every other plot line seems prestaged going into the game. I don’t think it would be so bad to have some variation where Kairi’s arc actually begins in the game itself. And it is at least set up that she is in fact training. The way they present her at the end of DDD even makes it seem like she could be a secret weapon. Since time is compressed where she and Lea are training, it’s likely they’ll be fairly advanced wielders by the beginning of KH3. So even though we may get to see some of the training process, I’m betting it’ll either be abridged or we’ll start their scenario towards the end of their training.
Like Mickey, I have no issues with Lea being playable. But I think Kairi should be the primary/initial playable character for story reasons. If Lea is the focus, Kairi will most likely come off as a bystander or like she’s just being dragged along by someone who’s treated her terribly in the past. By making her the character the player experiences the scenario through, it will be more like Kairi is in control of the situation, if that makes sense. She’s helping him, but it’s on her terms. Then as we progress, the events become much more centered on Kairi than she would have expected. Destiny is never left to chance and all that.
I think part of her motivation could also be to restore Namine, who would probably be the most likely to know how to help Xion return. That would give her and Axel a common goal to work towards. Plus, they both have pasts in Radiant Garden that could be touched on. I think Kairi still has buried memories from her time there that could reveal some important stuff. I think there’s plenty of story material to work with if Kairi is playable so that her character won’t be problematic.
Again, it’s a case where Lea could be playable in specific circumstances. Then once the scenario is over, we could go back and play as Lea and whoever else joins their party.
This is all just based on my hypothetical thinking of where the story might go. I certainly don’t think Kairi should be playable yet remain inconsequential. But I think it would be a nice natural progression between the main titles that Sora was playable in KH1, Riku was briefly in KH2, and then Kairi in 3. But I hope they just do what is best for the story they decide to tell. Until we know what that is, it’s hard to say who the best playable characters would be. At this point, it’s entirely subjective as it’s all based on speculation and personal opinions.
I do like your thought of having a playable character to represent each of the trios though. Honestly, at some point in the game, I’d like to see each of our protagonists playable.