It's a luxury problem IMO. CA lets us import units, buildings, landscapes, edit factions, interface, sound and animation. No other game gives us that possibility. We can always wish for more, and we will no matter what they give us. But that doesn't change the fact the current build allows alot already.

About the addition of factions. I am sure that you can add them to the exisiting batch. Think about it; the engine must know which faction eblems to use, in descr_models_battle.txt it is needed to have have different tags for each faction so that R:TW knows which skin to use.

And M:TW had the same limitations on the amount of factions and units.