I have been thinking.
If there ever will be a Kingdom Hearts 3, what worlds will there be?
Here is a few world ideas that would be cool to see in Kingdom Hearts 3.

Gears of War:
Now this would be quite epic, Marcus Fenix, Sora, Donald, Goofy and the rest of COGs fighting both Locusts and Heartless while trying to figure out the origins behind the Locusts. Wouldn't that be cool? Sora would get the Keychain, which is basicly a Keyblade with a chainsaw, how cool is that?

Dead Space:
This one would be perfect too. Sora, Donald and Goofy get a distress signal from the ship Ishimura and they just so happen to stumble upon the ship that Isaac Clarke and his teammates were onboard before docking with Ishimura.

BioShock:
Imagine Sora using Plasmids while helping Jack to fight Splicers, Big Daddies and saving Little Sisters.
Pretty damn cool, eh?

Duke Nukem:
Do I need to say anything about this? ;)

Party characters.
One other cool thing they should add is that the characters on the worlds I just mentioned would join Sora & company and help them fight the Heartless. They would be in the Gummi ship and wait for Sora to give the go-ahead for them to join the party.

Isaac Clarke: His engineering tools are extremely sharp and he and Sora can do this team-up where Isaac uses the Pulse Rifle while Sora fires a beam from his Keyblade and he is spinning around Isaac while doing that.

Marcus Fenix: Marcus' Lancer is deadly both at a distance with its machinegun bullets and at close range with the chainsaw. When he and Sora do their team-up, Marcus rushes at a selected foe and slashes at it with his chainsaw and then Sora jumps in from above and cuts the foe in two.

Jack: Jack's Plasmids and modified guns make him a deadly foe at a range. But he is also quite good at dispatching enemies with his wrench too. When Sora and Jack teams up, Jack uses both his Electroshock Plasmid and a fully uppgraded revolver at all the foes and Sora gets a shotgun and shoots any enemies that are in close range.

Duke Nukem: Duke can deliver intense amounts of damage on his foes with both words and guns. He uses his golden Desert Eagle when he is engaged in ranged combat and when up close, he uses his mighty boots on an unlucky enemies head or crotch. When he and Sora teams up, they stand back to back and shoots at all the foes on the screen while turning around in circles.