I think sewage would be a fun and potentially hilarious addition.
Some thoughts:
- Sewage should flow much like water or magma. For simplicity, both "#1" and "#2" type sewage products should probably just be combined.
- Thus constructions like waterwheels and pumps should be usable. For example, a map lacking a river could utilize this fact to generate "perpetual" mechanical power from a sufficiently active sewage system.
- Since some players would undoubtedly prefer not to deal with excrement, I'd suggest making toilets/bathrooms an optional construction, and the dwarves would only (need to) use them if they were available. If not present, then they just would never come up as a concern.
- Similarly dwarves should probably not be required to use the bathroom very often - say once every 6 meals or something. While this is perhaps unrealistic (though I hear dwarves have iron bladders) it would avoid the problem of dwarves constantly needing to use the bathroom.
- Also, for sanity, it might be best to consider toilet-use an optional "fun" activity like viewing statues currently is. In other words, don't make dwarves shit their pants if they can't get to the potty in time. I am darkly reminded of The Sims...
- Special furnaces could be placed over sewage streams, yielding an endless source of char-material and thus partially standing in for a lack of a magma source (but with probable additional complications such as miasma).
- Would add a useful purpose for soap.
- Automated sewage fertilizing systems could be created by plumbing sewage to be periodically dumped on farm plots, after "maturing" in an appropriate reservoir for some time first. Such developed "manure" should probably not produce miasma. A dual-reservoir system utilizing pressure plates and floodgates could possibly help automate the process.
- Inventory screen: feces smear (throat)
- Who would not like to see a sewage flood trap. Feces 7/7
- Drawbridge + sewage = hilarity. I also have to second the shitfall idea.
The biggest downside I see to implementing sewage is that it might lead to a profusion of poo-oriented DFMA videos, which might give potential new players a skewed impression of what the game is about.
And yes, this is my first post. Disturbing...