I'm quite sure something similar has already been suggested several times.
Sure, people have mentioned poop quite a few times before, especially given that it's one of the few essentials of real life that Toady has refused to model in the game. But no one has ever speculated the horrifying lengths that someone as obsessively detail-oriented as Toady could go to if he
did decide to model it, or combined it with smoke and disease to make your dwarves' lives that much shittier (pun intended). I started thinking about how filthy, smelly, smoky and generally unhealthy an underground fortress would be without proper ventilation and hygiene, and couldn't stop typing until I had brought it to life in all of its hideous, nauseating detail.
Edit: Plus, I made elves immune to diseases, no matter how little they clean up their own shit. How many other DF players have ever said to themselves, "You know, what this game really needs is for the elves to be
even more insufferable?"
Edit 2: I haven't even thought through all the possible ramifications of this yet. Already, one of the "spheres" where a powerful being like a god, demon or megabeast can have influence is called "muck." All that's needed is a few modifications to the in-game languages and hey presto!
The forgotten beast Filthspurts the Defecation of Diarrhea has come! A towering blob composed of feces. It has a bloated appearance and there is a foul reek about it. Beware its noxious secretions!