Personally, KH3 would have been much more interesting and exciting to me if it was a SRK game. Donald and Goofy can join Mickey in Aqua quest, and Lea trains on his own (sorry Lea). Like mentioned before, it’d be great exposure for the trio to have been a real trio, and it’d be fun to see the interactions of Kairi with Disney characters.
But thematically, I do see how the manipulation of Kairi and Sora‘s relationship has been a driving force of the series. This pining is Sora’s motivation (wholly are partially) in KH1 KH2 CoM and KH3. She has been used as a ploy by Ansem, Xemnas, and Xehanort, all three of the main antagonist throughout the series.
(An interesting point to note is the parallel between this and Nameless star... How this might connect to how Sora and the secret episode play out)
Ultimately, I could see Sora and Kairi never truly being together. It’s not what I want to happen, but it might be the case. If anyone has seen Onward, I kinda seeing it play out that way. Where the two love each other and care for each other, but in the end, they must learn to live without each other. More optimistically it could relate to the idea of forging a new path. This is touched on with the Hundred Acre Wood. Up until now their relationship has been defined by conflict, perhaps to be together they must build a new connection. Whatever the case, I can’t imagine either of them truly dying or going away forever and it wouldn’t be fair for one to get to stay in the RoL with all their other friends while the other is stuck in some other reality.