At this point, it would seem pretty likely we're getting a returning civ rather than a new one, so if you consider that we are, that basically narrows down the options to one of four civs realistically: Portugal, the Iroquois, Morocco, and the Hittites. Of these four, the Iroquois and Morocco would be the most geographically unique within NFP itself - We have the Gauls super close to Portugal and Babylon super close to the Hittites, but while we have the Maya and Ethiopia, they respectively represent Mesoamerica and East Africa, whereas we have no true representation of North America or North Africa in this pass. We usually see one ME/ME-adjacent and one African civ per DLC group (SP had Persia and Nubia, R&K had Georgia and the Zulu, GS had the Ottomans and Mali, and NFP has Byzantium+Babylon and Ethiopia), on top of the base game having 2 each (Arabia/Sumeria and Egypt/Kongo). The fact that we've already seen the BB combo in NFP specifically rules out Hittites immediately as that would be 3 ME/ME-adjacent civs, but we've never seen African get more than 1 new civ per grouping, so with the addition of Ethiopia, Morocco can probably be reasonably ruled out as well.
The basegame came with 8 European civs (England/France/Germany/Spain/Norway/Russia/Rome/Greece). This was followed up with Poland/Macedonia in the SP, the Netherlands/Scotland in R&K, and Hungary/Sweden/Eleanor alt in GS, for an average of 2 additional Euro civs every group. By this metric, we've already seen both our Euro civs with the addition of Byzantium and the Gauls (in the same pack, no less), which in theory should rule Portugal out as well since we've never gone above 2 additional Euro civs at a time past the base game. In contrast, we've seen exactly 2 Mesoamerican civs, both introduced in the DLC SPs (the Aztecs and the Maya). The SP is the only group of DLC to not include a new civ from North American proper, while we've gotten exactly 1 civ from NA in every other grouping, including the basegame (America in base, the Cree in R&K, and Canada in RF), and if you combine the two regions together we've gotten exactly 1 in every DLC so far. Looking aside from the Mesoamerican civs, we have a pattern of Country-Tribe-Country in the current 3 we have, as well as Vet-New-New, so it would stand to reason that the next NA civ to be included would be a tribe and also a returning vet to balance things out, which basically means the Iroquois, the Sioux, or the Shoshone. We are also due for another female leader, seeing as the SP, R&K and GF all had 3.
So due to the observable patterns, combined with overall requests amongst the fanbase both via the civ and the potential leader choice, the Iroquois/Haudenosaunee led by Jigonhsasee makes the most sense as a requested civ amongst the fanbase (that is from North America, a tribe, and a vet civ, which fits all established patterns with the other ones we've gotten so far), with a popular female leader choice (just like Tamar and Lady Six Sky) that is very historically relevant to the Iroquois and not just a diversity choice for the sake of diversity (Jigonhsasee being one of the three founders of the Iroquois Confederacy and on equal footing with fellow co-founder Hiawatha, who was the their designated leader in Civ III and Civ V)
However, all the above relies on us getting a returnee civ and not a new one. But going again off of patterns, we got 3 new civs in the basegame (Norway, Scythia, Kongo), the SP had a 5-3 split in favor of returnees, and both xpacs had a 4-4 split in terms of returning civs vs. new ones. We are currently sitting at 4 returnee civs (Maya, Ethiopia, Byzantium, Babylon) and 3 new civs (Gran Colombia, the Gauls, and Vietnam). If we're following the basegame + SP trend, then we're getting a vet civ and sending with a 5-3 split. If we're following the xpacs, then we're getting a new civ. So in the case that we're getting a new civ, they would still follow all the established patterns otherwise, so they'd need to be a North American tribe with a female leader. The immediate first option that comes to mind is the Cherokee led like Nanyehi, which are one of the most popular requests for a new tribe civ from NA, as well as a decently-popular leader request that is also not a diversity pick for the sake of diversity. Beyond the Cherokee, my knowledge of female leader choices in other popular new civ requests such as the Navajo or a PNW tribe is not extensive and I can't come up with any good ideas to suggest for them, but that doesn't mean they don't exist! But I also personally see the Cherokee as the most likely (Navajo second, due to the unique espionage-based playstyle that they can bring due to the Codetalkers) due to their popularity as a request amongst the fanbase when bringing up new NA civs.
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