A Sioux or Iroquois return seems more likely to me? The Sioux especially who haven't been in the game since Civ 2 would certainly *feel* like a new civ (and not necessarily a pure horse raider civ - I'm willing to share my own design to underline this point if needed.)
Question though: if Firaxis were to go for a Muscogean tribe, why wouldn't they just go for the Creek? The Creek call
themselves "Muscogee" (which could serve as the name of the civ itself) and while their leader choices might be less interesting than Choktaw/Chikasaw isn't there enough overlap between the four non-Cherokee tribes to join combine them into one single civ with a
maybe Cherokee UU. Or would that be on the level of Welsh-speaking woad-painted Brittanic Queen, who received faith from trees and had a Gallic UU and an Irish UB and who settled modern Irish, Cornish, Scottish, Breton, Mancunian and Welsh cities? (nb.: kind of funny how this post also lowkey exploresThe Concepts Zaarin Has A Pavlovian Fear Of. sorry brah)
Secondly, if a 5CT civ were to happen, what would the UU and UI be? The clear contender is the Cahokia Mound but that's already a CS tile improvement (and I don't see Cahokia being included into a Muscogean city list - not if Firaxis has the intention of being historically accurate

). So kindly run me through the options, please. The idea of a 5CT civ intrigues me but in the words of our mighty internet encyclopedia, this article has reached a stub and needs further expansion.