Idea one: Desert with good possibilities to make pyramid/temple. Agamas tileset is superb, but it misses sand, and something to put on sand would be nice too. There could be extra tileset (Or in same) ability to make cool sand storm at desert/temple ruins.

Idea two: Lava cavern with lava. There is quite alot lava cavern tilesets, but many seems to be lacking lava...

Idea three: Ice mountain/canyon with possibility to make snow storm. Not christmas or cartoonish.

Also, idea of underwater base sounds cool.

And all of these could be "serious" tilesets. Many tilesets are too cartoonish... And yes, I know cartoonish is easy to make compared to ÿbêr cöól tileset.