1. To this day, I am irritated at the linguistic inconsistency in the series. We have Jadwiga, but not Caterina or Pyotr. The Ottomans really should be called the Osmans. And some of the city lists... It's not that I mind exonyms. The devs do have to keep in mind what market they are developing for, and keep the bar of entry pretty low by using more resonant terms and concepts. It's mostly the hopping back and forth between exonyms and endonyms that bothers me; it reduces immersion. Pick one and stick with it, please.
Being both a historian and a bit of a linguist, a little consistency would be nice. Heck, ANY consistency would be nice . . .
2. I don't see a Tribal/City-State mechanic going very far mechanically without becoming excessively complicated. Civ isn't meant to be a historically accurate simulation; just a historically suggestive virtual board game. I don't think differentiating internal tribes is what's lacking here, elsewise nearly every civ would fall under that criticism. Bavaria is so different from Brandenburg, guys, and I'm just so offended the devs neglected to show that in Germany's design.
I agree, it cannot be done comprehensively without becoming complex and annoying. But, for instance, there have been 'city state/tribal' Mod Civs in V and VI that had at least a nod towards the concept: either the city by its name or by your choice, for instance, starting with a Bonus for a certain type of District, or yield (extra Production, Religion, Gold, Favor, specific Military Unit Production, extra Charge in their Builders, etc), so that not every city in the civ started out identical. I rather liked that system because, of course, it meant that the cities became less distinct as the Civ developed, which is exactly what has happened historically.
Although, having lived in both Bavaria and Brandenburg (well, West Berlin) I have to admit that they are still very distinct in culture, language, and general attitude . . .
3. Depends on the overarching thesis of the game. Since VI is focusing on culture heroes and personifications to more strongly embody their respective peoples, Sitting Bull and Hiawatha are very strong candidates to fit in with the rest of the cast. Under a different design philosophy, of course I would support different leaders, for the same reasons that I'm thankful for abandoning the same tired roster of Washington, Napoleon, Hannibal, Caesar, Hammurabi, Maria Teresa, Elizabeth, and Catherine. It gets stale. (Now, is this new design philosophy likely? I'm dubious, because the franchise is sold on resonance, and what resonates with most people is what few things that stuck with them in their high school history class. But still, nothing wrong with hoping for even more radical shifts in the franchise.)
To some extent, although the exact amount can be dated, any commercial game/enterprise has to consider its audience/market, and the market for a game based however loosely on history has to consider the abysmal lack of basic knowledge on the subject, or the prejudices learned about the subject. I don't think that should completely limit our choices, though, and in the majority of Five in Civ VI now it hasn't. While Victtoria, Robert Bruce, Theodore Roosevelt, Pyotr are well known - partly because of recent books, TV shows, and other non-game media about them - how many fo the general English-speaking population could have identified Tomyris, Tamar, Seondok, or Poundmaker? I think there's room for variation, if the leaders are well done and evocative of some aspect of the Civ that is readily Identifiable to the general gaming customers.
4. Well, in the broadest sense, given that we have America smack in the center of the Atlantic seaboard and Canada wedged up against the Great Lakes, the Powhatan and Anishinaabe don't really "fill" anything. And mechanically, the plains peoples would be fighting against the Cree for design space. The biggest "gaps" on the map are, as others have observed, the Pacific coast, the Southwest, and the Caribbean. HOWEVER, in another installment starting fresh, I could see the Powhatan or Anishinaabe replacing the Iroquois, or the Blackfoot replacing the Sioux/Cree, even moreso if for some reason Canada is left out.
Let me throw out an alternative concept here. Instead of worrying about TSL, as in Geographical Location for a Civ, how about looking at Variety in geographical Type of Civ. Instead of which earlier Civ is in the same area as modern Canada, look at which native civs are in northern forest, coastal marsh, Great Plains, Mountains, desert, etc. Within this framework, you would have some Civs (Cree among them) that actually thrived in distinctly different different geographies: great plains and northern forest. The Lakotah, in fact, had elements living in marshy forest (modern Minnesota) and the great plains, and in an area like the North American southwest you have a group like the Comanche that ranged from desert to great plains and tribes like the Apache and Ute that lived from desert to mountain terrain. That would give, so to speak, a different menu to choose from in selecting and modeling Civs.
Just a thought.