I doubt there's going to be a lot more Civilization 6 content, but if there were to be more civilizations, these are the ones I think I'd like to see most.
Sioux lead by Sitting Bull
I'm sure Zaarin has an argument against this, and I get it. They're known for losing. I'm not exactly pining for the confederacy to be added. That said, I can't think of many civilizations and leaders better fit for Civ 6's "big personality" gimmick and popular history perspective than this combination. I mean, unlike some people's picks they're at least known for doing something.
Romania lead by Vlad the Impaler
In a game with Brazil, Australia, and Canada, I think it would fun to just go all the way on modern civilizations. You know, just 'cause. Such an opportunity might not come around again.
Israel would be great but has little chance of getting in, so let's go with the fandom's other obsession from the ancient near east. The Hittites, by merit of Biblical mention, fit that pop history idea like a glove. People might not think about it right away, but you mention them and people know what you're talking about. They have a clearly definable "gimmick", early access to iron or strategic resources more generically, which would make it fairly easy to have a design which gives the Hittite fans what they want. Come on now, with natural disasters in the game, Mursili's Eclipse could be a leader ability with a prophetic or omen element to it.
Similar to Mexico, this is a well known modern nation that people are likely to think of. There's a lot to draw from on the Italian peninsula which Rome just doesn't cover, and I think it would be a good candidate for a civilization with multiple leaders. This wouldn't have to be a unified Italy civ, it could be a blob civ similar to Greece- only instead of Athens and Sparta it's Genoa and Venice, or Turin and Florence, or whatever else, really! It'd all be pretty fun, I imagine Italy would make a very flavorful culture-focused civilization.
I think we need another central asian civilization- it just feels a little empty, you know? Part of me wants to suggest the Timurids or something, but I honestly think we have enough cavalry rush civilizations at this point. Another part of me wants Tibet, but I think there's a China problem there or something? I was considering Afghanistan, because that sounds like it would be cool and sounding cool is basically my main criteria for this list, but I decided to go with Iran instead. This would represent the Islamicization of Central Asia and Iran is the only Shia majority country I know off the top of my head- accompany their inclusion by dividing Islam into Sunni, Shia, and Sufi. Add Anglicanism and Oriental Orthodoxy too while you're add it, just Balkanize all the religions. It's the sort of talk that gets me thinking about an expansion with a religious schism mechanic, but that is extremely unlikely to happen.