sports, kickboxing and such. Emphasize that even artists are athletic.

Ethnic yet modern designs (ancient cultures' patterns "modernized").

animals

Dragons and other fantasy creatures :3

Treehouse, as in the children's television channel...? I've been out of the loop for awhile. Not my vote anyway, unless it's Miss Spider's sunny patch friends. bounce rocks! Apart from being a bed bug, that is ):
I really like the idea of insects.
SteamPunk-Insects would be cool, but maybe have insects in split into two sub categories for that and include Conceptual-Insects and there is the contrast everyone loves to see. For the ones who want to take it an extra mile, Conceptual-Insects vs SteamPunk-Insects as a third sub category in case there is a major influx of the one vs the other.
Always equals fun.

Tranquility/serenity in nature

fat farm animals

Future minimalistic design.

Clean colors with strong outlines.
Abstract variations maybe too

My Little Pony for my opinion. x)

How about steampunk time machine? yess