This might be a tall order, and I'd love to help work on it (once I figure out how to mod!).
I would like a "neutral", non-civilization aligned unit that civs could build. Pretty much like pirates, mercenaries, or the "privateer" unit from Civ 3. They could invade other territories but not cause a war, but should probably be weaker than regular units so that players don't exploit them and launch unofficial wars.
Ideally, I would like an infantry unit, a cavalry or mobile unit, and a naval unit in each age, along with a gunship. Graphically, the unit colors would have to be made neutral somehow, but the units wouldn't have significant changes over the existing models. So, a mercenary swordsman would simply be a weaker, neutral colored swordsman model.
Let me know if you have any interest, and I can work out stats. And thanks so much for all the great work!