Unsure how many people here have tested the beta for this but there is a RPG version of DOOM 1 called DOOM Fall of Mars (Doom FOM) which i find very fun at the moment
This version of doom reminds me of Diablo (both game play and graphic wise) in this RPG version you travel around UACs Phobos base and other places.
Like most RPG your character is the Marine and you gain weapons, Armour and other items and also go up different levels.
Similar graphics to doom 1 and Diablo but with more game play, Check it out and test the beta (the beta is very CPU hungry so make sure you run the Beta with Compatibility Mode for Windows98 (some times happens in Windows XP happens all the time in Vista as its a bug but setting to WIn98 solves this problem)