This has been mentioned before but there hasn't been to much said about steam, other than toady does not want "steampunk dwarfs". Steampunk is more of an aesthetic element.
Steam already exists in the game but does not really do anything other than exist.
It could be said that steam engines are not a pre-1400's tech. But simple steam deviced actualy date to 1st century AD. The only difference between ancient steam devices and industrial revolution ones are complex pistons. Simple pistons existed pre industrial revolution.
To keep it from being the end all be all simple power supply, there should not be a workshop called a boiler. Instead it would be more dwarfy if the player had to construct a boiler chamber via mining one out. (Weather using lava as a heat source or burning fuel.) Then mining outlets or using pipe sections to channel it. As for the actual conversion of steam to mechanical energy. This could be done while keeping it in style with the world (not having complex pistons/turbines), by using it to power windmills/ pumps/ water wheels.
A player would have to manage steam levels as well, to prevent boiler explosions (sounds very "fun").
This all could be used in 100's of ways. Traps, power source, cooking and cleaning (for medical dwarfs) to name a few off the top of my head.
This could all simply be achived by making steam and upward falling liquid that dorfs can't drown in (or mabey they can if the steam gets to thick).
It would even fit with traditional fantasy dwarfs. In alot of various fantasy works, dwarves used complex machines and steam engines.
TLDR
ADD USEABLE STEAM