All my ideas up until now revolved around having to switch as little parts in and out for making them space-able, but seeing this made me slightly more inspired.
*warning: incoming brainstorm session*
The helmet idea is excellent, those horns would probably work better than the standard ones. The gun and jetpack are excellent too to get the heavy look down, perhaps if you could put additional horns on the jetpack that could look deadly. Basically if you can get the Bull faces, torso's and swap out some of the grey parts for black ones (I'll be doing this when I rework my own after the forum battle is over as well) as well as take the binoculars and neck bracket off so it doesn't interfere with the chest logo I think you should have a nice troop design, but I trust in your ability to think up awesome. Or you could keep the neck bracket (helps to make them look badass tall and armoured) and don't bother getting the torsos. How about sticking a horn or a disc on it? Or both if it's possible? Down-or up-facing horns on the chest? Just making up stuff in my mind here, I can only reach my Lego in the weekends so you'll have to check for yourself if it looks like it's worth anything
If you want to get more analogy with the fantasy army well, their shields probably wouldn't work anymore but round shields could do the job of taking the place of the Phalanx. Or their shields may still work, dunno really. The axes are great!
I'll be looking forward to seeing this evolve.
Icon hardsuits, anyone?