I prefer that they take their time and complete the whole game...The worlds in KH are part of the story, it would be unfair to sell worlds as DLC and leaving a bunch of players without knowing that part of the story (since a lot of players don't usually buy them). I actually don't see how DLC can fit on KH without triggering a good part of fans, but I think the best approach would be special items like keyblades or a boss battle.
Given how nomura talked about DLC and how that leak awhile back kinda came true, there is a possibility of it happening.
I hope not and it doesn't really make sense... If I buy a game, I want it to be complete. So no Story-DLC, that'd be awful. But even if they release some worlds as DLCs that are not important storywise... Why not keep them for future games? That'd be a complete waste.