Making siege weapons and working on a big, complex alchemy system. It's gonna be big. It'll start out slow (much like my modding of it), but the end result (assuming you do all this convoluted shit based on authentic Medieval alchemical texts) will be making a Philosopher's Stone, which you'll be able to use to transmute metal, heal wounds and cure diseases (of course diseases are in), create living homunculi, and possibly even resurrect the dead. I think the homunculi might require DFHack.
Once all that's done (and the non-Western civs have their religious buildings/interactions), I'll be able to release my big medieval total conversion project.
I'd like to also add a demon summoning/blasphemous ritual system, that would possibly have tie-in/overlap with the religion system (basically any sort of demonic ritual would permanently damn the practicioner so they could not perform any religious building reactions), but I'm terrified of coding.
I'm going full medieval with this shit. I've already long since removed all non-European/Middle-Eastern/North African plants and animals, not to mention I've removed the ability to play as dwarves and elves (they exist (maybe) but are much more mysterious and inhuman; you have to play as a human (from one of seven cultures) in this mod). Three temperate cultures, three tropical cultures, and one weird cave-dwelling arctic culture seems like a good amount, doesn't it?