I didn't want to enter details like units, heroes, etc. on this because of the reasons the Lord of Gifts and DieWalküre pointed out, and because we don't even know if they would be an agressive or defensive faction. When I helped stirring up this discussion on ModDB I was more worried about other contenders for the two faction "slots" to be filled after the remaining announced factions are released. In fact, I still am interested in any other possible factions that could be implemented, besides the Men of Darkness, because I can think of none.
In the case of no other strong contestants, I think the ideal thing would be for Rhûn and Harad, or Easterlings and Southrons to use more general names, to be separate factions, but I may be wrong in this. Lore-wise it makes sense given how big a territory these people occupy and how diverse their culture is, having many separate tribes and peoples under the Easterling and Southron umbrellas. These diverse peoples could be turned into units, but maybe I'm exagerating, and there isn't that much material for two whole factions. There certainly is for at least one, not to mention artwork I've been digging.
In either case, even though I said that I don't want to enter into details, I had this general idea of a mobile wainrider outpost or building. Given how these people live, travel, and wage war in their wagons, I guess it would be interesting and unique in the mod to have a wagon-building, or maybe a building formed by many wagons, that is capable to be moved around and relocated in the battlefield.