Just some thoughts ......
Why can't there be two types of theme challenges. Like this:
1. Restricted. Cliffs -- submit tgd and image files that generate your cliff and make the tgd available to all the forum members. I believe this would filter out objects that aren't part of the Terragen package, like World Machine terrains, xFrog vegetation, etc. Anyone could download the tgd and try to render it and not get "file not found" errors (the tgd submission should state the Terragen version.)
2. Open: Anything goes -- contestants can use purchased or self generated files (models.)
Danny could spell this out in the rules (restricted or open.) I guess, in this case, it would be "a cliff(s) or cliffs scene -- restricted or open.
Those restricted tgd's would be a nice learning asset!
I have a question about prizes. Is it legal to submit a scene with a purchased item, say a tree, that doesn't have a commercial license (*and win a prize?)