One of my Players might play as a LE Diabolist (Sorc->Diabolist). Im thinking his God wants a Foothold in the River Kingdoms. Great RP/City Adventure with Cultists showing up, etc. Also thought it might make for fun if they Showed up in Book 6 to find the Big Bad Dead and the PC and His God (Demi god?) in her place. Opinions?
I wouldn't do the last bit of replacing the big bad with the PC and his patron. A LE diabolist isn't going to like the big bad in Kingmaker (she's sort of almost diametrically opposed to such a character, actually) so it's a natural fit.
I could foresee a number of things going wrong here, though, but they're all the same "Evil parties/games are a bad idea" tropes that have been rehashed a million times.