First idea:
Most Portal RPs I've seen take place in the Portal 2, post-apocalyptic sort of setting, with the famous GLaDOS and all. While I'd be perfectly fine with that, I was thinking of maybe doing an Old Aperture one, which might be harder, but certainly fun. If not Old Aperture, it could take place at around the time of GLaDOS's activation, so there'd be lots of running around and screaming and neurotoxin.

Second:
Hunter-gatherers, probably those in Northern Europe or around that area. Any universe, but preferably Cro-Magnon/Homo sapiens sapiens, although it could take place during the short period of co-existence with Neanderthals. I just love that sort of roleplay and I could totally dream up a culture and all that to follow in character.

Third:
James Cameron's Avatar? I really loved that movie and I'm hoping a few other people would be interested.

And then maybe even a superhero-type roleplay minus the superpowers, like Kick-Ass.