A World War II secret war game would be the best for me. 1930s - 1950s a B.P.R.D. like group Vs paranormal/occult/Thule society nazis. GoGo killing Hitler in like 1952!
Last time I was at the FLGS, I saw a Hellboy game, but I didn't look too closely so I don't know if it's any good. I'm not even 100% that I'm not imagining it.
It would be cool if there was a steampunk western fantasy game. The various races from fantasy-- elves, dwarves, orcs, etc set in a wierd science fiction west.
I didn't find a single original idea. It's pretty typical third party stuff, but they couldn't decide which setting assumptions to make, so they made all of them. It wasn't really put together with much thought, so you'd have to go through and pick though what does and doesn't work for a given game. The most dissappointing part was that the steam technology wasn't really fleshed out that well, which was the main reason I bought the book. You'd be better off with d20 Past if you really want d20, otherwise I think there's a GURPS supplement (I haven't tried it, though).
Between that and their OGL Cyberpunk, I'm pretty much soured on anything Mongoose.