My wife would vote for plumbing, with a big "slop sink" and even a shower. Hell, she would like me to even put a toilet out there!
Depending on what level of work you plan to do, make room and power for welders, tanks etc. Also maybe a solvent parts cleaning cabinet.
Lots of ventilation too, besides heat and a/c. I put in a big electric fan (like an attic gable fan) in the ceiling area that exhausts to outside to extract fumes and exhaust. I bypassed the t-stat wiring and put it on a 60 min wall timer. When fumes are building up, I just put it on for 30 min and clears everything up real well.
