we had a water units contest recently, and thats pretty similar to 'submerged'
clockwork as been an option as a theme before, but its never won, and clockwork is fairly similar to 'vehicles'
plants? mmmeh, i dont want a doodad contest, if we have to make a 'animated' plant 'UNIT', then we're just going to get 30 venus-fly-trap submissions.
space, we just had a spaceship contest for pmc2
worker unit... i just dont like it
sorry i didnt include all your ideas but you did say
"There is not much chance that I join, so you don't need to take note of this post. " :3