Okay, my proposal
Maya, Portugal, Ethiopia, Byzantium, Assyria, Vietnam, whatever native american civ, and either whatever incarnation of Italy or whatever incarnation of Moors/Maghreb.
Maya is a must have, Ethiopia is the only "obligatory" Subsaharan civ (no Zulu are not obligatory), Portugal is major missing European country with massive historical importance and very unique exploration vibe, Byzantium has just enough crazy fans like me who want it forever.
Assyria instead of Babylon, because
*It's still Mesopotamia
*Incredibly important, massive civilization
*Which is underrepresented in series, unlike Babylon
*Sumer has more similar vibe to Babylon that Assyria (Ziggurat, city list etc)
*It is the only clear "warmonger, agressive expansion" style civ on this list
*It's awesome
Vietnam because it's damn popular and would widen South East Asian roster.
Whatever Native Americans are necessary because I don't like tribes getting into the game as civs but American collective guilt needs catharsis.
Whatever incarnation of Italy OR whatever incarnation of Moors/Maghreb. I'd say just "Italy" but while I'm 100% on board with giving Italy "Greek treatment" of making them united civ composed of amalgamation of medieval/renaissance city states - it's still a little gimmick and I'm not sure if Firaxis would like it. And the capital overlap with Rome would either require some weird capital or gimmicky solutions such us "if Rome in game then make Florence capital of Italy" or whatever. And Moors are my second choice because I am crazy medieval_islamic_phile, al-Andalus was amazing, and introducing medieval civilisations of Islamic Spain and Maghreb would be the second best thing this year after a certain vaccine.
As for alternate leaders - alternate Chinese leader is the most probable one imo. This is the civ with the biggest asymmetry between historical "size" and amount of 1 leader per game next to India AND Qi Shi Huangdi is not new to the series so that'd be an opportunity to introduce some fresh face AND it has (obviously) massive Chinese players market. Not to mention a metric ton of great leaders to choose from.