An interesting lesson people will learn is that what we, with child-like wonder, believe will be cool, will turn out to just be a Disneyland of propertyhodgepodge which will not make for an interesting game per-se.
But those that will persist through the awkwardness, might find gold. Zelda Four Swords was a banger after all
Right - you can't let players edit everything. Second Life had that and it was a digital junkyard.
I'm working on a game right now that tries to find a nice balance between player made stuff and gorgeous assets they can't change.