1. Italy: being a nation of great historical influence and the only one of the great European nations that has never been in the game completely, its inclusion is completely overdue. We can have a Renaissance Italy led by someone from Venice or Florence and the fact that they are organized by city-states should not be an impediment to such a thing, since we have Pericles (Athens) leading all of Greece and Lady Six Sky (Wak Kab' nal) leading all Mayans.
2. Ashanti: an African civilization with an extraordinary political, military and economic organization, Ashanti is another overdue inclusion. Kumasi has been present in Civ6 since the base game, we cannot deny that the devs have their eyes on the Ashanti. That way, I wouldn't be surprised to see it in Civ7, even in the base game.
3. Amazigh (Berbers): the Maghreb is a very interesting and important region, so it would be a glaring omission not to have at least one representative from it. Amazigh would be my choice here, but I wouldn't complain about something more specific like Morocco again.
4. Muisca: perhaps the most solid option for South American natives after the Incas. Muisca could fill an interesting niche that would not be redundant with any other civ in the region: ore trade with a moderate war focus.
5. Argentina: this one competes directly with Gran Colombia and Mexico for the slot as a Hispanic colonial nation and, being South American, it's the main candidate to replace Gran Colombia. Argentina stands out enough on the continent to deserve inclusion in Civ, and would possibly be a cultural Civ with some commercial bonuses. I wouldn't complain if they chose Mexico, though.
6. Navajo: to be honest, I don't have much knowledge about the native people of North America. I always thought, however, that the Iroquois should be a staple in the game, with the second slot alternating between some civ from the American Southwest. A third slot could be held by someone from Canada.
7. Swahili: this one, or Somali, could occupy the slot of maritime commercial African civilization. Personally, I would like this slot to be a staple and for Swahili and Somali to alternate in it.
Honorable mentions: 8. Armenia (very interesting history that deserves to be represented in Civ), 9. Taino (my first choice to represent the Caribbean. Haiti is also interesting) and 10. Romania (personal interest, although I wouldn't want it to replace Hungary).
7. Amazonian (eg Marajoara)
Unfortunately, there is not much to create a Marajoara Civ. The leader would be fictitious or not historically appropriate. The list of cities would be predominantly modern Brazilian. I don't even know if there is enough information for a unique unit for something other than "Amazon Warrior".
Marajoara would be interesting for a city-state, however: "Os Camutins" could be a trade city-state (agrarian would be more appropriate, I hope they include this type of city-state in Civ7) that would allow the construction of the Teso improvement on floodable lands.