My current one is kind of weird.

[I]Nightmare Before Christmas[/i]-based with Lock, Shock, and Barrel aged up to teenagers, and then adding other characters their age to play off of. Generic funtimes, right?

Then add in some [i]Rise of the Guardians[/i] elements where Pitch Black is the One True Bogeyman, and the Man in the Moon chooses holiday leaders.

All right, now we have crossover elements. Cool.

Then realize Halloween Town's [i]never[/i] had teenagers before (seriously, watch the movie: everyone's either a child or an adult with no in-between), and that teenagers are rebellious fucks with the time and energy to get involved in the way adults can't because of their obligations. Have the trickster trio realize they've been pawns all their lives, have the other teens see the shit going down in not only their town, but the other holiday worlds, and the next thing you know, the whole plot goes from "wacky teenage hijinks" to "we are literally banding together to take down this world's equivalent of god because screw that."