Kingdom Hearts 3: Stuff Happens. Stuff Doesn't. They All Eat Cake in the End.

MY GAME IDEA!

World 1: Traverse Town
BLAH BLAH BLAH TRAVERSE TOWN BLAH! Sora, Donald, and Goofy find out Traverse Town is cursed by another dimension close to it, and they must venture to that dimention by opening a portal activated by all 12 dark hearts. They try clearing the area of all the heartlesses and fight the Lord of Demon Warlocks, and get the first Dark Heart.

World 2: Galaxy Far Far Away (Star Wars)
Sora stops by Phineas and Ferb's Dimention Traveler Shop and take their ship to the first dimention, the Galaxy Far Far Away. They crash their ship into Mos Eisely. The Dark Heart Tracker points to the Death Star, and they hear the Millenium Falcon is going to the Death Star, so they sneak into the ship. Once they get the Death Star, they sneak out, and go into the ship with temporary party members Han Solo, Chewbacca, Luke Skywalker, and Obi-Wan Kenobi. They help save Princess Leia, and find Darth Vader, who is pocessed by the Dark Heart. They fight him and get the Dark Heart out of him, but they can't pick it up and they are forced to escape.

Later while the Rebel Alliance tries to destroy the Death Star, Sora, Donald, and Goofy have to dash in, grab the Dark Heart, and leave before the station blows up. They get it and travel back to their dimention after repairing their crashed dimention travel.