I wonder what kind of enemies he will have?
I was already thinking about a pilot and this is what I come up with:
Setting: Las Vegas or Los Angeles (City of Sin or the City of Lost Angels, I think it fits for a start)
I thought of a kind of pilot. I wonder if I should put it here or try to write fanfic out of it.
Hunters are hunting supernatural activity in the area. Suddenly a they encounter a human but superstrong and immune to everything they throw atthen Cas jumps into the mix and saves the hunters with enochian bloodspells but he messes up and one of the hunters gets capture.
Something like each person he saves from evil gets him that much closer to Heaven. In which case I see his enemies as being demons, angles he crossed in the past (such as an also fallen Raph, etc...).
I see him ally himself with some hunters here and there, perhaps an also fallen Balthzar (or perhaps he's the Hevenly contact who gives him his assignments and progress reports).
Maybe he can even pick up a pet or something, something like a witch's famliuar but more angelic like manybe a bird or something.
Perhaps Atonement would be a name for the show, if somewhat dire.