I'm working on writing a paper on Tecumseh and Pushmataha for one of my classes for my master's. In the process I'm studying the Shawnee, and the more I study the Shawnee the more I think they'd make a horrible choice for Civ. Not because they're not interesting--their history is fascinating--but because their most defining attribute as a culture was their mobility. They wandered from the Great Lakes to the Carolinas to Louisiana to New England and back again. They'd be very difficult to portray as a coherent civilization. The best I can think of is a very gimmicky design where they settle lots of one-tile, one-pop cities that can move and double as districts.
There are a lot of nomadic groups throughout Earth's history, and I don't see a way to easily incorporate that into Civ VI, unless you have a game mode devoted to civilizations that have no permanent settlements.
For Native American civs, I still think the Wyandot (Huron) people or the Métis would be excellent additions to the game, and I don't believe either has been featured yet in any of the previous civ games.
I was pleasantly surprised when the devs added Gaul (albeit with a focus on Belgian part of Gaul), so maybe the devs can eventually add the Huron or Métis to the game.