I recall having read somewhere that additional liquids other than water and magma may be implimented into dwarf fortress in the future. As a suggestion for a type of liquid to introduce, how about Sewage?
Do bear with me while I explain~
First of all, defication is a fact of life, so why not add it? Steping in (or worse, submerging in) raw sewage could generate unhappy thoughts or even create disease once that is also implemented. Along with the containment and disposal of the sewage through sewars or cisterns this would offer quite a chalenge and certainly be rather 'fun'.
Often when trying to make a fortress seigeworthy one will try limit any entrances, leaving the chief concern Wagon access, wood harvesting and fresh water - all things coming IN to the fortress. What if you also had to worry about taking something out of it as well?
I imagine toilets could work something like a well - an item of furnature built over a channel with channel depth affecting quality. The item could also define a room as a public toilet or as a private one for nobles, etc.
Once inside some manner of sewer system the waste could be channeled or pumped to a remote location where it does no harm, dumped into a river or chasm, used to fill a moat or even as a nice water feature for the slums. River dumping could include risks such as water poisoning and killing off fish.
Before the fortress is on its feet properly or if thier are no toilets constructed, the dwarves could just choose high ledges or designated zones to do thier bussiness.
So, thiers my argument in favour of Sewage as a liquid - what do yous think?