Idea giver reporting for duty!
Hmm, let's start from simplier (Seems you can't make custom mesh as I guess) things...
Infested Marauder/Firebat - Well, I guess those would be really useful for the modding society.
Armed civilian - pretty much, a marine with it's mech like torso, head and legs/hands replaced with civilian's (Well maybe except for some parts, like torso and legs). Imagine how sueful this would be as some sort of militia/light infantry!
-Some light terran vehicle variation. You know, Hellion merged with one of those civilian vehicle. imagine one of those cruisers, Shikoto coupes or whatever modofied to suit the combat.
Hmm, kinda hard to come up with ideas for zerg/protoss now, but maybe I'll get some ideas for you later!