I would like to join in on this, if custom models is accepted. My other project isn't going to show for a while - and this is a great way to advertise each modder here (as well as the opportunity to show some work). We have quite a few maps up here today and I think that it can become confusing for newer users to know which mapmaker create stories suited for them.
I see that you have solved the hub level in a cleaver way!
While reading through the thread I thought of something like "completing a map gives the player a piece of a puzzle which he physically lays out until it becomes a picture; perhaps some fan-art with each of the mapmakers signature on it".
Everyone has their own vision of this. I think it that trying to tie this project to a strong story would defeat its main advantage: the uniqueness of each map. Instead make the player experience several high-quality maps in sense of scripting, exterior and scare events. As everyone can focus on a single map it would probably come out pretty well done.
Creating a map to fit in story-wise would put a damper on creativity. Instead I suggest that each map presents some sort of smaller story.