They can easily make a new, unique plot like they did in KH1 with a lot of worlds.
Sora arrives in Andalasia in the middle of the woods next to a stream. He comments on the beauty of the world. He then hears a yell coming from a near-by clearing. He rushes over there and finds Edward fighting some Dream Eaters, Nathaniel cowering behind a tree, and Sora joins in with the fight. Fight's over and Edward thanks Sora for his help but claims he had it all under control. Edward starts to walk about but then one of Sora's Dream Eater companions hopples around and Nathaniel yelps. Sora then says he'll stay with the Prince and his servant.
Go through a couple of areas where you can fling off trees and at one point you have to swing your keyblade around some branches so you can get to the top(Nathaniel stays at ground level). Edward then looks for his destination up there and tells Sora about his journey and stuff. They then talk about the Keyblade(as everybody Sora comes into contact with talks about).
Edward then spots the house. You do some more flips and spins to get down.
A few more areas where you fight Dream Eaters and you hear some trees crashing. Sora and Edward turn around and see the Troll running towards them. Nathaniel runs off again.
You fight the Troll yadda yadda yadda.
Then you reach the house. Sora thinks he spotted something back in the woods and he leaves without Edward noticing. Edward sings about true loves kiss and Giselle pops out of the window.
Sora walks through the forest and the Witch and Nathaniel are watching him from their tower(he's in one of those things where you watch people though water or something). Nathaniel talks about how he almost had Edward stopped but the boy interfered. The Queen gets furious.
Now, I know that sucks but that's my idea of how they might make a world out of the first level. Of course, it's really short (I'm not a writer) so professionally done they'd have been able to expand it plus Riku would take the palace so there is more there.