From doom we got Hexen and Heretic which tried to be midevilish things with hints of RPG elements.. I wouldn't exactly call them RPG's so much as slightly leaning that way. Strife, the last doom engine based game to be sold commercially, is more in line with modern action RPG's what with having dialog, things you had to retrieve, stealth potential (that would get ruined by those stupid green lines that would trip the alarm no matter how stealthy you've been til that point,) and even branching paths.
Except Strife isn't, technically speaking, the last commercially available game based off of doom. It is the last game using licensed code from Id Software, but there was another game that was developed using an interesting process involving piecing together how the levels are put together so doom editing software and map-makers could be used for the actual level designs, and presumably some other stuff for all the non-doom bits and blobs.
This game is Amulets and Armor, and it didn't have a publisher. No, instead it was sold direct to order using a similar shareware model Doom, Jazz Jackrabbit, Duke Nukem 3d, Strife, and a bunch of other PC games had in the mid 90's. Looking at the Let's Play series points at a game with multiple classes, multi-player co-op, varied locations and gear. So what happened?
Then it got picked up by the two guys that made a let's play series out of it and somewhere along the line they'd gotten hold of the guy that coded the thing. Legal snarls were fixed, rights were obtained, and this beast of a game is now legally free to download courtesy of the developer himself for any and all to play while he goes through to try updating the dos version and plan for an update to the engine to let said game have some new life as freeware that is playable on modern systems without the need for dosbox. OK that's not the whole truth since there is a windows version from the same site, but unless you're playing with a windows box, and because I'm not I don't know what sort of fiddling needs to be made to get it to work, you'll be better off using the dos version and a copy of dosbox.
Also because while the level architecture is based enough around doom that you can build for it using a plugin for doombuilder it is not enough like doom to work with zandronum or boom or whatever source port you like blowing up demons with.
There's plans down the road to redo the entire game to work off lua scripts, use .png files, use .zip archives for packaged material, and in general allow anyone to edit anything. This excites me since while I am a fan of 'classic' games, and this is something old enough my netbook has zero problems running as is, it's got some bugs, texture alignment issues, suffers from level design that would seem questionable to modern gamers, and a whole host of things that it will be nice to see get fixed in this Community Release..
Source is available on github if anyone wants to have a look, play around with it, or try helping out. The official site has downloads for windows and dos versions, along with links to a wiki, community forums, and is a good general starting point if you want to give this old thing a spin.