Turning Fanfiction into Regular Fiction

Over the past four years, I've been writing a very long fanfiction (you might almost call it a novel) based on The Nightmare Before Christmas. The problem is that what I've written has taken on a life of its own. I've fleshed out enough secondary characters and created enough secondary plot lines that the whole work really doesn't resemble the claymation anymore apart from the fact that it contains a really tall, lanky skeleton who's in a complicated relationship with a ragdoll.

I would like to turn this into a regular fiction novel. The only problem is I am horrible at worldbuilding. I can expand on existing worlds, but creating my own in which I have to describe settings and character appearances is something I find very difficult. (Usually plays are mostly dialogue and detailed character descriptions are discouraged so people can cast more easily).

In short, I'm wondering if anyone can help me with worldbuilding and if it would be ok for me to post some of my fanfiction novel so I can work with real examples.