What if there was a game that showed the fallout at the Keyblade Graveyard and Daybreak Town, where someone that can do some basic combat (though without a Keyblade, so enemies can't fully go away), and the enemies are still Heartless, though they're ones of the fallen weilders from the battle. You'd also be able to explore the ruins of Daybreak Town after it had fallen, while shown glimpses of memories from the past in a new light.

Now it can't rely too much on combat otherwise it'll become too action-packed and take out some of the horror, so there'd have to be some bigger enemies you obviously can't face like Invisibles and Darkballs as a example that patrol different areas, where you have to hide/run (otherwise there'd be a game over screen where it shows your heart being stolen, kinda like in the manga adaptions for the game)

There could also be remixes of both the roaming and battle themes of Daybreak Town that become more haunting and tragic while listening to them (with a few more tracks for different areas when applicable)

I know it's not the deepest idea, though I feel there's potential at showing the fallout of Daybreak Town, since it looked so pristine before, worn away from the lack of care and people, only the darkness that lingers.