AU: Western would be to write something that is in the western genre, which is generally associated with frontier expansion back in the American west (things like cowboy movies, building the railroad, etc) but could be taking place on another planet (ala Firefly) if it still has the same feel of cowboys, leather dusters, gunslingers, exploration, new frontier, settlers in a new land, etc. I can provide some links with more info on the genre if you link.

Re: the sequel question, yep, that's fine!