Here's three KH games I would want:
A small, $30 side game where you play as Mickey on his quest to meet Minnie, become king, and start his journey to being a keyblade master. The game would start in a vastly expanded Timeless River where you first encounter Pete, then would go to the Mysterious Tower where he first meets Yen Sid and begins apprenticing, leading to newly expanded areas of Fantasia that we haven't seen before, then would go to Country of the Musketeers where we would encounter Pete yet again and Minnie for the first time, seeing a lot more of Mickey's interaction with his two best friends in the whole world, and ending in a vastly expanded Disney Town/Disney Castle, where the town has been taken over by some sort of darkness (maybe the ink from Epic Mickey?) and there's dark versions of Disney characters everywhere. The game ends with Mickey establishing the Cornerstone of Light in Disney Castle and sealing out the darkness, with Minnie's powerful light magic being the only thing that can initially seal the Cornerstone away from darkness. Mickey is crowned King and marries Minnie in the epilogue, leaving the castle at the end of the game to take on "a new challenge" - the events of Birth by Sleep.
The second could be your idea for a YX game, I would LOVE that! Also, Young Yen Sid NEEDS to be in this series!
The third is a Birth by Sleep Vol. II that happens post-KHIII, which I've talked about in the past and still want to talk about more in the future.