Europe:
England (Elisabeth) - Don't know which one Elisabeth , I let the firaxis decide. This civ simply cant be skiped from vanilla.
Rome (Marcus Aurelius) - Prob most important civ in western people history. I would even say them with china, greece and egypt is most important civs thoughout human history.
Greece (Alexander) - Alexander is too important to skip historicaly , he comes either as whole greece leader or as alternative one. Whole greek culture and history is imposible to skip.
Russia (Catherine) - Dont see a civ without Russia. Could be different leader tho.
France (Napoleon) - same as Russia.
Vikings - in some form.
Asia:
China (Taizong Tang) - one of most important civs in human history , even now China is one of most important countries in the world.
Japan (Toyotomi) - famous for they're samurais, budism, sengoku era.
India (Gandhi) - Im sure that we will get always India in vanilla and nukes lover gandhi

Vietnam (Lady Triệu) - need some new guys.
Africa:
Egypt (Khufu) - I still hope we get ancient leader with ancient cities for civ6. As for civ7 egypt is one of the most important civs in human history.
Madagascar (Ravalonna) - need some new guys.
Middle-east/Fertile crescent:
Arabia(Saladin) - Islam. And great history.
Sumer or Babylon.
Persia (Darius) - should be in vanilla and not some DLC. Shame for civ6.
North America
USA (Jefferson) - nothing much to explain here.
Aztec(monty) - really not fan of DLC for so important civ , keep dlc for alt leaders or someone like nubia.
South America
Brazil - for sales I guess.