As ProjectX2 mentioned, we've seen people try a couple of shared universes, and they haven't worked well, but there's no reason not to do it again. Two things that I ask:
1) Keep all discussions civil. It's okay to disagree, but don't let it get unpleasant or out of hand.
2) Stories for this universe need to follow the site guidelines for ratings, subjects, etc.
Zombipanda's comment about the "extra" levels of this universe echo one of the old rules of writing: if you introduce a gun in the first act, you must have fired the gun by the end of the third act. (A paraphrase, but I'm sure you get the point.) I'm not a strict adherent to this rule myself, but it does keep writers from cluttering their stories with unnecessary items. I see the "Heaven" and "Hell" levels as existing in case anyone wants to use them, but they are not required elements in any story for this universe.
Is this what you had in mind? If so, you might want to leave them undeveloped until someone needs them, and then get the universe's participating writers together to hammer out the details so whatever is written about those levels/inhabitants is consistent. After all, if you're writing a story set in our real universe, there are plenty of stories that can be told with no supernatural elements at all, and others that venture into those realms to varying degrees.