So... Z is a RTS game, which only a single tech in various colors. Adding new Z related factions would just be a clone of this faction but in a different color. The only other thing I could think of would be to also make them speak in a different language, since the first and second game actually do have the dialog lines in a number of languages but that's about it.

I don't know if it's a good idea, though. I could end up being "clutter" factions.