Not everyone likes Flowmotion in KH3. Besides myself, I know a few other friends who dislike it as well. Regardless, Square as a company is big and rich enough to test things by themselves instead of releasing such a weird project like DDD again, and that was the point MATGSY made (which I completely agree with).
Anyways, I still think Kairi should be the lead of the next game, but I think Kingdom Hearts is missing in personality lately which the latest replies seem to agree. I think we need more of what the first game had back then with the unique artstyle, each character kinda doing its own thing, having their own sense of style. Lately everyone has a Keyblade and flashy moves and it could get stale. I think looking towards the Persona franchise for inspiration could be cool, with more unique quirks to the game's design in the menus and personalities, and maybe some sort of character-interaction system evolving from what D-Links and Summons were. Let us chat with the other Guardians of Light through the gummi phone to get to know them more giving us fun interactions and, as a reward, new abilities, skills, commands and so on.
I would like a Kairi-focused game to be somewhat inspired by what FF-X2 did but not as much; the main games since 2 already have this with the forms which work similarly to the outfits and class system of that game, but imagine befriending Riku through his "buddy-system" giving Kairi a Light-Firaga (aka Dark Firaga, but with Light magic) and, when maxed, the Oblivion Keychain. Befriending Aqua gives us Mine-Square, Xion gives Ars Arcanum finisher, Naminé unlocks KH1 passive abilities or something.