Seems like we have.
Here is my conspiracy theories:
American Staples: America, Aztec, Brazil, Inca, Maya. Returnees from previous game: Iroquois as geographically close to Canada's start. New: Argentina, Haiti, Muisca, (replaces Canada, Gran Colombia, and Mapuche), and another western "horse raider" such as Navajo/Apache or Comanche.
Central/South/East Asia Staples: China, Japan, Korea, Mongolia, India, Indonesia. Returnees: Vietnam, and either Siam or Khmer or both if Burma doesn't make it. New: Mughals (separate from India centered more around Afghanistan/Pakistan), Burma/Tibet?
Middle East: Arabia, Babylon, Carthage/Phoenicia, Persia, Ottomans. Returnees: Assyria/Sumer or both.
New: Parthia and Armenia (as Scythia and Georgia replacements)?
Europe Staples: at least England, France, Germany, Greece, Russia, Rome in base game. Duh.
Dutch, Byzantium, Poland, Portugal and Sweden in DLC/expansions. Maybe Spain if not in Vanilla. Returnees: Norway/Denmark for Vikings. New: Ireland (Celts), Some form of Italy, Goths (Classical Era), Franks (if Alexander can have his own civ why not Charlemagne?), Romania if Austria/Hungary don't make it.
Africa Staples: Egypt, Ethiopia (maybe Classical Era inspired over Nubia), Zulu. Returnees: Mali or Songhai, possibly Morocco. New: Berbers if no Morocco, Angola (over Kongo), something more modern instead of Ethiopia like Nigeria or South Africa?
Oceania: Australia, Maori, Hawaii/Tonga/Samoa (as the seafaring Polynesians)
I think I maybe got to about 57 so add in another wildcard somewhere for 58.
My speculations:
Europe (2 more than Civ6)
Returned ones from Civ6: Byzantine, Dutch, English, French, Gaul, Georgia, Germany, Greece, Hungary, Macedon, Poland, Portugal, Rome, Russia, Spain and Sweden.
Replacements: Denmark (in the place of Norway) and Ireland (in the place of Scotland).
New ones: Austria (I think they will be back in Civ7, but I don't think they will replace Hungary) and Venice (or other form of Italian representation).
North America (2 more than Civ6)
Returned ones from Civ6: America and Canada (yeah, I think there is a high possibility that Canada will be back in Civ6, it's a typical diplomatic passifist civ and tundra meme).
Replacements: Iroquois (in the place of Cree).
New ones: Navajo and Cherokee.
South America (1 more than Civ6)
Returned ones from Civ6: Brazil, Gran Colombia (GC was a very demanded civ and it has presented the game to many Latin players, I don't see them abandoning Simon Bolivar in Civ7) and Inca.
Replacements: Argentina (in the place of Mapuche).
New one: Muisca (although Guarani is likely as well, either as a replacement for Mapuche or a new one).
Mesoamerica/Caribbean/Central America (same as Civ6)
Returned ones from Civ6: Aztec and Maya.
There is a chance for Haiti, but I personally don't think that is very likely.
East and South Asian (1 more than Civ6 )
Returned ones from Civ6: China, India, Indonesia, Japan, Khmer, Korea and Vietnam.
New one: Siam (I'm betting on industrial or renascentist Siam. I don't think a Mughal civ will happen in Civ7, but they can add multiplus Indian leaders).
Middle east and Central Asian (1 more than Civ6)
Returned ones from Civ6: Arabia, Babylon, Carthage/Phoenicia, Persia, Ottomans.
Replacements: Parthia (in the place of Scythia) and Assyria (in the place of Sumer).
New one: Hittites.
Africa (1 more than Civ6)
Returned ones from Civ6: Egypt, Ethiopia, Mali and Zulu.
Replacements: Morocco or Berber (in the place of Nubia) and Ngola (in the place of Kongo).
New one: Ashanti (Madagascar is also likely).
Oceania (same as Civ6)
Returned ones from Civ6: Australia.
Replacements: Hawaii (in the place of Maori).
There is a chance they can add one more in Oceania, so the set would be: Australia (mainly for TSL reasons and marketing), Hawaii (a new flavor) and Maori (a fan favorite in Civ6).
Overall, my set of civs for Civ7 is very similar to Civ6.
I'm actually okay with America and Brazil because I feel like they've become something different from their mother country; Americans were, in fact, already very culturally different from the English in England by the time of the Revolution. While my personal preference would be to have few or no leaders/civs more recent than the 17th century, I think there are reasonable arguments for including America and Brazil that don't apply to Australia and only apply marginally at best to Canada. Of all the civs in Civ6 I think Australia is the hardest to justify (followed very closely by Scotland in its current design, albeit not Scotland ipso facto).
Can we consider (Gran) Colombia culturally distinct enough from Spain? I think so.