>gorgeous assets they can't change.

How are planning to block that? If they can create Zelda MMO, they can modify your game as much as they want as well, right?

Oh. Well - if it's an MMO you control what the world may contain via the server. They can add stuff to their local version, sure, but others won't see it unless they want to.

They can create their own world but then they need to get people to migrate to it.