Idk. Maybe poke around Indians, Japanese and Koreans. A bit of three kingdom shenanigans might also be fine, as that part was featured in some historical battles as well as lots of Chinese TV shows and games.
We dont need any more new civs. It is already quite bad we get another two in the next DLC.
If i had to decide, then i wouldnt add the two new civs into the DLC. I would put Battle Royale or stuff like that in its place. Maybe add some capture age features, so we get better spectator features. That would result in a much better DLC.
I only say remove the new civs, add something else. You can still have new campaigns. Even ideas like having different skins for every units depending on the region of the civ is something that we could add for SP.