Exactly.
The point was that small tribes or countries, won't usually be in conquest games, because, well, they are too small to conquest.
I mean really, where would Liechtenstein or Andorra raise an army large enough to conquer the world?
They don't have ports, so, a Naval fleet is out of the question.
They don't have the pop. for Large scale economic development to sustain the maintenaince costs of many cities. They basically represent a city, in Civ terms.
So, seperate Nat Am. tribes wouldn't make since either.
Now, if you wanted to suggest 2-4 groups of Nat. Am. tribes as Empires, that would be conceivable. Such as a North-East, South-East, South-West, North-West regions.
Then, use the tribes in each area, as the city names for each region, but, there wouldn't be enough of a difference to justify it.
It would be better to have a different leader with different traits to explain a warlike version of a tribe, or a Philosophical leader to signify a more peaceful version of a tribe.
There was an attempt by some tribes to unite, but, nothing long-term or perminent was sustained. Some were never conquered. The Seminole Indians are here still in Florida. They were not conquered by Spain, or the U.S.
One could argue for an Indonesian Empire. There was rumor their early boats made it to south America.
They could have an early UU galley perhaps, and a UB like the Fetoria, with +1 gold from water tiles (perhaps changing the Fetoria to a Harbor for the Portuguese).
Several seperate islands with similar cultural backgrounds.
The difference would be to introduce this more sea dominent peaceful Empire to the game with a different cultural background.