You should check into the doom modding community. I don't tend to play many rpg style wads but I know they're out there. Some use the doom engine but the game is pretty much unrecognizable as doom (called total conversion wads). Here's one article with a list.

One called Hedon got an official release and is on steam etc now

They also make some hexen levels (I saw Wrath of Cronos being recommended).