So, assuming these leaks are accurate that implies that, A). the Maori are more or less confirmed, B). we're getting more improvements for sea tiles, C). some kind of ship, possibly a xebec, and D). Eleanor of Aquitaine is involved somehow.
Man, I hope we don't get Eleanor of Aquitaine as a leader for France. We already have one controversial French leader in the game, we don't need one more (though these days I'm more or less cool with Cat; she's a deserving leader, though there are more noteworthy figures in French history). Charlemagne? Charles Martel? Louis IX? Henry IV? Louis XIV? Napoleon? Charles de Gaulle? Even Napoleon III? I'm not saying that Eleanor of Aquitaine isn't an important or noteworthy figure—she absolutely is—but all these figures are much more worthy of representing
the entirety of French history than her. Hoping she's a scenario leader, like others or saying, because if she's an actual alt for France, I'll be quite disappointed.
Still, the Maori are definitely overdue for a Civ representative (technically there's Polynesia, but they were an awkward, albeit fun blob Civ represented by a Hawaiian king; hardly a solid representation of the Maori), and the inclusion of the Ottomans is
way overdue, if the xebec is meant to represent a Barbary corsair. If we're to assume that it's going to follow the format of R&F—which is to say, eight new Civs and one new Leader—then my predictions will be this:
Maori: I really don't know enough about Maori culture or history to make a call on the leader or abilities, but the UU will surely be a Warrior replacement. It might work like the Maori Warrior from Civ V, or to adopt the cannibalism thing, make the unit gain strength after a kill. (this is probably OP, but it sounds cool at the least!)
Ottomans: Yeah, I'll be honest: Suleiman is really the only choice. You could include Mehmet II as a siege master-themed leader, but c'mon. Picking Suleiman as the greatest Ottoman ruler is like picking Marquee Moon as the best Television album; there is no other choice. His ability would certainly be something related to administrative reforms, and maybe with an added cultural bonus too. Maybe even stick the Barbary Corsair as a UU for him as well. Because you also practically
have to include the Janissary as a UU as well.
Inca: Before we move onto any more Native American Civs, we have to get the Inca out of the way as the other "great empire" of the Americas. Pachacuti is the natural choice here. The Terrace Farm should absolutely make a return, and their UA should reflect upon their sophisticated record-keeping system, infrastructure, and labor.
Austria: I might get some eyerolls over adding Austria as an EU rep, but to me their importance to European history is just too large to overlook. Charles V, Maria Theresa, or Joseph II would all be solid reps. Give them their Hapsburg marriages, and honestly, bringing back the Coffee House might not be a bad idea.
Portugal: Yet ANOTHER EU rep for the game, but we really can't not add Portugal, especially if this is the last expansion for the game. Henry the Navigator, Manuel I, or John II are probably the most thematically appropriate rulers, but you could add Maria I if you just want a fun personality and gender diversity. Make them the obvious trade and exploration Civ that they should be.
Ethiopia: We need an African rep, and Ethiopia are probably the most glaring absentee. You could bring back Haile Selassie, dig back to use Zara Yaqob, or you could even use Ezana or Kaleb from the Aksumite kingdom. A religious Civ to be sure.
Thailand/Sukhothai: Southeast Asia could use a rep, and since we have the Khmer to cover the Western part of Indochina, why not bring back Sukhothai? If you were to tweak Father Governs Children to work with the new city-state mechanics, you could have a really unique and fun Civ to play with. Ramkhamhaeng is probably the logical choice, and I know everyone wants to see his smug mug back in Civ.

Iroquois: We need a North American rep for this expansion, and yeah, the Iroquois are probably the most prominent one not included. If we don't get any other expansions/DLC, this does mean we go without a Great Plains rep for our Native Americans, but I simply feel the significance of the Iroquois shouldn't be overlooked. Plus, it gives FRX a chance to do the Civ right this time. Rather than go with a mythical figure who may or may not have actually existed—we already have one of those in Civ VI—let's add in Joseph Brant/Thayendanegea this time around.
Alt-leader: This choice would have to be a representative that not only is prominent, but seriously changes up the way you approach the game. My predictions assuming we don't get Eleanor of Aquitaine, which IMO would be a travesty, would be Ramses II for Egypt (conquest over diplomacy with Cleo), Darius I for Egypt (more city-building and economy over Cyrus' conquest), Louis XIV for France (cultural/militaristic Civ instead of Cat's unique espionage niche), and Harun al-Rashid for Arabia (cash flow/trade over Science; or you could integrate the Baghdad Golden Age into his design somehow, even though it's kind of represented by the Madrasa).
A lot of these are returning figures, but to be fair the base Civ VI and R&F inclusions have a lot of wonky inclusions; even with these, we're missing a lot of mainstays, which is a little upsetting. I hope this expansions adds more Civs so we get a broader representation, but I doubt we will. Some of my other wishful thinking Civs are Babylon, Sweden, Hungary, Mali, Morocco, Maya, Muisca, Sioux, Cherokee, Chinook, Shoshone, Carthage, Kilwa/Swahili, Vietnam, Assyria, Italy/Italian City-State, Ireland/Celts, Byzantium, Denmark, Israel (which will never happen, but a man can dream), and some South American post-colonial state—Argentina, Gran Colombia, or Chile would fit the bill, though Chile would overlap with the Inca in a TSL map, so maybe not them.
As for gameplay, I don't really have too many demands: just improve diplomacy, add more buildings and units, fix up the tech tree, and improve production scaling—though that last one might be fixed by buildings.