Since we're on the second round of the G&K civs, I personally would like to see:
Austria: Domination on Continents
Byzantium: Diplomacy or Science
Carthage: Something on an Archipelago
Ethiopia: Domination
Huns: Diplomacy
Dutch: Culture or Diplomacy
I had fun with the Babylon Deity "Any Victory" scenario. Maybe we could do another Deity level challenge which lets you choose any path to victory.
Also, at some point, an Immortal or Deity Diplomacy game where the AI's are all genocidal CS killers would be fun (Temujin, Attila, Askia, Montezuma, Bismarck, Catherine, Napoleon, Oda). Maybe you could have extra city states. It would be amusing to see which city states would actually make it to the end....