Some ideas. I suck at making tilesets, so I don't mind sharing them:

- A mountain village with waterfalls and trees and small brick-houses with wooden roofs, background with sunset or night or something. Hmm, Kinda like those landscapes in Unreal (like Harobed village or Spire village etc.)
- I like the mine/cave level idea. With carts and tracks so you can create a DKC mine cart type level.
- A temple ruin with vines and lots of plants, big boulders, Sunny-sky background
- A very very very very VERY modern tileset, with flashing stuff everywhere, so you can make a propper space station and ship. - A chemical laboratory with green stuff oozing through pipes, wires hanging around which can electrocute you, some sort of doors that can open and close.

Right, those are the tilesets that I would find extremely usefull (if well made) for the single player episode I'm building @ the moment.