Which random famous historic figure is about to show up in Civ7 as a leader? Sacagawea? Machiavelli? Sammuramat?
The inclusion of Tecumseh/Shawnee pretty much rules out having Sequoyah/Cherokee in the base game at least, which is a little disappointing to me because he could have made for such a cool Scientific/Diplomatic leader and the "Not necessarily heads of state" principle this time around is probably the best opportunity to include him. But if wishes were horses and all that.
Machiavelli could be an interesting one for sure, and raises the question of, with Rome being exclusive to the Age of Antiquity, whether we'll get an Age of Exploration Italian civ, and which of several potential options they would go with. The three main options I see there are:
Venice: It's the only post-classical-era playable Italian civ we've gotten in the series so far. However, I don't think it could play at all like it did in Civ5 with the Cities/Towns mechanics as they are in Civ7 (plus, imagine doing the REX that Rome's build pushes you towards and then having every city you've established in the Antiquity become a vassal with no build queue in the Exploration Age!) So, possible, but I don't think it's the most likely.
Papal States: We can surmise from Shawnee having a missionary replacement UU that religion will be involved in the game to some degree beyond just pantheons. That it's not a Victory Condition makes it seem more like a supporting mechanic. Having an Exploration Age civ go all-in on Religion might make sense, but we don't really know enough yet to make an educated guess about this.
Florence: This seems the most likely to me, but I've been pretty biased towards wanting Renaissance Florence in the game for ages now. It just makes so much sense as a cultural powerhouse that defines and is defined by its era. It's also kind of the only one that could fit for Machiavelli as a leader, though I still feel like Cosimo or Lorenzo de'Medici is more likely for that.
I think it's very possible that we get Jeanne D'arc for France, since there's virtually no way that France isn't in the base game and Napoleon is locked behind some shenanigans, but then again, with leaders untethered from their Civs, it's just as possible that Napoleon is the only French leader and that if you don't unlock him then you're just stuck playing France with Amina or whatever. I also think it's very possible that we'll get Argentina as a Modern Age civ this time around (and given the limited number of base game civs and Inca's near-confirmation of inclusion, I think we can expect Argentina or Brazil to be the Inca's successor civ, for better or worse) but if we get Argentina, I think they'll probably be led by Eva Perón.