To the stygian abyss with mecha, I'd like to see steam powered mechanics. Even without a complex control system (like a steam engine), it'd be fun to harness the power of magma + water... Sure we know that clouds of steam scald and cook flesh, but if it's created in an enclosed space with a small escape path, you could have steam jets, steam cannons, steam powered pneumatic rams (so you have to scrape goblin paste off the ceiling). You could set up a pressure plate linked to a floodgate that allows the magma and water to mix.
I could see the need for some new mechanic's items, like a pressure relief valve to prevent excess steam from leaking into dwarf-inhabited areas (in case you miscalculated the precise mixture for your ram). Maybe pistons as well. But just the things needed for "macro" engineering projects. Perhaps this would be the best method for mass transit/portage power. Probably only used for object transportation (could be a bit hot, wet, and/or violently accellerated for livestock or delicate goods). Plus, they could act like transistors for you psychos who want to use these sorts of things to build macro-computers (link a steam pressure sensor to a floodgate, etc., etc.).
Just some ramblings from a dabbler who has yet to even dig to the chasm!
Ratbert #CP#Z
"For FUN and HONOR!"