There is more info to extract from NFP initial announcement we could use to predict new civs. It's like solving an equation with many unknowns.
First let's look at new game modes requirements. Pack#1 game mode requires GS - climate change and disasters for Apocalypse precisely. Pack#2 game mode requires at least R&F - governors mechanics for Secret Societes and loyalty mechanics for cultists precisely. Pack#3 game mode requires again at least R&F mechanics, so something to choose from: governors, loyalty, emergencies, reworked alliances, timeline. Packs #4, #5, and #6 game modes require only vanilla mechanics. So no climate changes, no disasters, no power, no reworked strategic resources, no diplomatic victory, no greviances, no competitions and no earlier mentioned R&F mechanics for Packs #4-#6 game modes. These are requirements for new optional game modes, not for new civs/leaders.
The only pack with special requirement for new civ/leader is Pack#5 which would be R&F for obvious reason - we know already from the leak that's because of Kublai for Mongols (and probably for China). Maybe Vietnam would use R&F mechanics as well - e.g. Trung sisters: one as a leader, another as an unique governor? That leaves us with the rest of new civs/leaders all based on vanilla mechanics. So far neither GC nor Maya, nor Ethiopia uses other mechanics than vanilla's. So civs/leaders in Packs #3, #5, and #6 will use too only vanilla mechanics. In lights of this some assumptions posted in this thread would be wrong - at least partially - e.g. Portugal can neither get an unique governor for colonies because that would require R&F expansion nor get exclusive Great Admirals/Explorers
with every new era (like GC's CGs) because that would require R&F again. Of course Portugal could get an unique Grão Navegador even four times in the game by using vanilla roads progress mechanics when new era technology or civic is discovered.
Four new civs we expect to get in September, November and March will be vanilla by game mechanics totally but they could be still extremelly unique like Maya or Gran Colombia or at least unique-ish like Ethiopia. There is still vast potential to use in vanilla mechanics for new civs/leaders. We have already civs/leaders terrain-or-feature depending: Russia and Canada - tundra, Mali and Nubia - desert, Brasil and Kongo - rainforest etc. Even unworkable for other civs mountains can be workable by Inca. What about unworkable oases and ice? I still see a gape for Inuit-like civ working ice, snow, and tundra or for Berber-ish civ working oases.
Next thing - every extension brought some new content and about 75% new civs/leaders somehow connected with this new content. R&F gave us: reworked alliances and Cree; loyalty and Zulu, Mapuche, and Netherlands; timeline and Georgia, and Mapuche; governors and Korea, and Mapuche. GS gave us: reworked strategic resources and Hungary, Canada, Ottomans and reworked England; enviromentalism and Maori; competitions and Canada, and Sweden; diplomatic favors and Sweden; power and reworked England. In NFP so far we got only one new civ connected with some new content - Mayan focus on farms and new global farmable bonus resources - maize. Neither GC nor Ethiopia has connections with new content so far. There will be no new resources requiring Irrigation so no connections for GC's haciendas with new content. So pretty weak usage of new content by new civs at this moment. We could still expect more civs (like 4 of 5 incoming ones) to be connected with new content - e.g. none of already revealed civs in NFP uses Diplomatic Quarter in special way. Let's look now on new content to arrive: Pack#3 - nothing special, Pack#4 - new Great Persons, Pack#5 - new district and 2 new buildings, Pack#6 - nothing special again.
Let's focus on packs #4 and #5 for a while. Grand Persons (except for GC's CGs) are generated by specialty districts. There are 3 categories of districts in Civ6: green ones (multiple for civ and for city, non specialty = non generating GPs), purple ones (singular for civ and for city, specialty but non generating GPs YET!) and differently coloured ones (multiple for civ but singular for city, specialty = generating GPs). New GPs from Pack #4 would be new entities for already existing category or for some completely new category. If the last then how these would be generated? By the new district from Pack#5? No, because of timing. GPs from Pack#4 have to be generated by already existing districts. All (but one) of differently coloured specialty districts generates just single GP category. Only TS generates 3 GP categories but all 3 of them doing the same - culture and tourism. I dont expect any of differently coloured specialty districts other than purple ones could be used to generate alleged new GP category in Pack#4. Besides the new district from Pack#5 could be another (third one) purple district in the game, to use together with Government Plaza and Diplomatic Quarter to generate new GPs. Otherwise the new district from Pack#5 would be a green one because a differently coloured specialty district couldn't exist without its own category of GPs. So I expect (and I hope) for new GP category from Pack#4 generated by 3 (2 already known plus the one from Pack#5) purple districts - from now fully specialty districts (like differently coloured ones).
What would it be? My guess - Great Statesmen or Great Diplomats focusing on spying, improving relations with major and minor civs, national prosperity, defense and expansions etc. So what about the new district and its 2 new buildings from Pack#5? I can imagine some kind of Financial Area or National Treasury place with buldings working towards economy but in different way than by commerce. Maybe by issuing currency? Then buildings could be Mint (for coins) and - more advanced - Print House [casa de papel] (for banknotes). It would be used in Pack#5 for its game mode which the leak identified as an "alternate economy" game mode. Maybe we could get an optional victory type - economic or financial one? There is in vanilla a suitable game mechanics already - religion mechanics which could be used almost directly: currency instead of religion; marks like $ instead of religious symbols; monetary policies instead of beliefs; spreading by cities with purple districts instead of cities with HS; dealers/brokers/counterfeiters instead of missionaries/apostles/gurus etc. Whoever spreads their currency to all civs wins. And Pack#5 comes with Kublai. It's worth noting he issued paper money known as Jiaochao that was known then for Europeans thanks to Marco Polo.
Maybe let's look for other optional possible game modes at the end. Anton assured us all of incoming ones will be historical, not fantasy ones. From Pack#4 I would expect a game mode focused on espionage with special units performing covert actions on enemy soil - i.e. on improvements and roads, and by damaging enemy units during peace, and finally by killing (yes, actually killing) enemy GPs (leaving actions in districts for regular spies as currently). As someone guessed MoorTires could mean More Riots

As for Pack#3 it should be a game mode using R&F mechanics - different than governors probably. I hope for a civil war feature (maybe based on loyalty mechanics) similar to that from first Civ when you conquer enemy capital and then their civ split into two civs beeing at war to each other. Imagine Rome breaking into Roman civ and Byzantine civ. And BTW, at least one civ/leader in every expansion was from ancient/classical era (e.g. in R&F that was an alt ancient/classical leader for India). Why Byzantium and not Babylon? Well, I expect a second season pass so let's save some iconic Civ civs for it

And game mode from Pack#6 based on vanilla mechanics again could be... I don't know. Your turn to guess.