If they release the bare .WLD files, they're okay, but anyone putting out whole .S3D files is asking for it unless they replace *all* of the other files inside. There are lots of public-domain textures out there and software for making more, but the sticking point will be the placeable objects. Unless someone can figure out how to place them (or at least release a totally empty one that anyone can use), custom zones won't look like much.
Edit: Here's a program that people can use to make their own textures:
http://www.texturemaker.com/
It costs USD$70, but it might be worth it if you're seriously considering making your own zones...
WC
P.S. With the scripting language, there's a real opportunity for people to accelerate the zone development process by writing object scripts and releasing them. The more kinds of scripts, the faster custom content can be created.
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