The issue with adding Native American civs is the same issue with adding Polynesian/Australian civs.
The fact that they were pretty cut off from the rest of the world in the time period aoe2 is based on.
I don’t think that is an insurmountable barrier but you have to find the balance between making them authentic and making them fit into the aoe2 tech/unit framework.
It isn’t like aoe4 where you can have completely unique and different civs.
I think some more civs in the Chinese area would work well and go down well with the playerbase there (although due to the thorny issue of Tibet and the one China principle I can see why they don’t).
Definitely room for more African civs and Middle Eastern civs potentially.
Aside from those I think American/Australasian civs are the next contenders.