But on that note, how would you all feel about Mickey Mouse's House of Villains being how the full story ends here? The Disney Worlds are on a time loop. Every time they're close to victory the heroic light shines through and they're defeated and their story starts over ready to be retold without their knowledge.(VHS Rewind/DVD starts over)
Maleficent discovers the truth on one of her rewinds and decides the only way to change her fate is to flood the universe with darkness. Her evil alone isn't enough and so she travels the worlds to find other villains and a true source of darkness, which leads to her discovering the Doors to Darkness, but she's not able to open them. So while Xehanort believes he's the one pulling the strings it's really Maleficent and the movie villains.
So KH4 is the House of Villains. We travel to worlds we've already been to as well as new ones to fix their stories and liberate them from the darkness. Each movie will have a distinct change. An example would be Ursula becoming Ariel's sister and seeing Prince Eric first while Triton is the Sea Warlock whose own dark feelings towards the surface world and the royal family created an almost dystopia Atlantica. No joy. No singing. Just despair. Kinda like Scar's Pride Rock.