For me, on the other hand, having to build a working sewer/cistern would be way too much work, when my dwarves are sleeping in the mud, starving to death, and fending off giant eagles with their bare hands.
On the other hand, a drowning chamber for elves that uses crap, and being able to imprison goblin babysnatchers under the latrine would be awesome.
And on the other other hand, hopefully the new workshop system, more accessible raws, etc., will allow a latrine mod.
First, you'd need some way of periodically generating a very high priority "go to the latrine" task for every dwarf, at semi-random intervals. If a dwarf is prevented from completing this task for a long time, whether because of the absence of latrines, locked doors, long lines, etc., they get a very bad thought.
Every time the "go to latrine" task is completed, 1/7 level of waste is produced in the central tile (which needs to be built over a channel). Coming into contact with this liquid should make dwarves very unhappy.
And you could have another workshop to which waste can be brought in buckets, and turned into fertilizer or whatever.
Just need a custom workshop and a custom liquid. The hardest part would be randomly generating tasks like that. Though if the game is made moddable enough, it could be made similar to how dwarves decide when it's time to eat and drink.
And actually, it would be very beneficial to be able to mod things like that, anyway. You could make critters which crave food at different intervals, or that require a specific food at certain times, or that periodically feel the urge to do a particular job (like strange moods, but regular instead of once in a lifetime).
Footkerchief, you're good at recognizing redundant stuff. Has "opening" the task-generation system been proposed? If not, I'ma start a thread.