For a moment, I thought this was going to be a Doom mod for DF (Doom Fortress). Basically, it would be in similar vein to Doom Rogulike, except adapted for DF. Dwarves would be marines, Humans could be, well Humans (surviving Hell on Earth), Elves could be UAC operatives (they get killed alot anyway).
As for the other races, they could be the different classes of monsters that you face throughout the game, like an imp race, or zombified humans/UAC/Marines, and so on. Megabeasts can include Cyberdemons or the big demon head thing with John Carmack's head inside that spawns the new enemies at you in the final level, except this time, that sucker is mobile. (Adventure Mode; "John Carmack's head" is a target you can hit).
Imagine this big fella trampling the landscape and spawning minions as it charges towards your fort. Even though in ASCII, just the thought of this thing in full size attacking a fort is a scary concept. In adventure mode, of course, a headshot with a rocket launcher will do the thing in in a few shots.
You'll never expect that deep "deep underground" (Hell IN Earth) would be safer than the surface.
This mod could share some traits of Wastelands in regards to having guns and all. Getting BFGs to work properly would be a bit of a challenge. I think those could only be made uber-effective if they (including the ammo packs) are crafted out of adamantine. Plasma guns (again, better materials for more effect, like the adamantine) in great numbers can spell overkill with a well-trained army of marines.
DISCLAIMER:
Copyrights and yadda yadda yadda; more what I wanted to say is, I lack the modding skill, but I like the concept of Doom Fortress. If anyone is willing to take a crack at making this, feel free to do so.