I think my speculation may vary based on whether they’re considering a Final Frontier after New Frontier. Granted at this point FF will only happen if NF does well... since we’re already in a “new frontier” in terms of post-XP DLC. I think my guesses need to be tightened up.
I still think old favorites are coming first. We’ve been asking for them for years and it makes sense to push them through now if FF isn’t guaranteed.
My list of missed favorites for the longest time has been: Babylon, Maya, Byzantium, Ethiopia, Portugal.
I’d be very surprised if we didn’t get those (and we already know we’ve got two of them, Maya and Ethiopia), although it’s possible the Babs might tap out for Assyria.
After that, it’s anybody’s guess. Gran Colombia took one new civ spot, so maybe two more to go. (And hopefully plus eight more from FF later!)
Maybe we’ll get some of the other missing civs: Assyria (if not taking Babylon’s spot, though I really don’t think we’ll get both in the same season), Austria, Morocco (I’d love to see al-Andalus), Siam, the Iroquois.
I don’t think we’ll see odd-ball Venice, blobby-style Celts, repackaged barbarian Huns or last minute substitute Shoshone again. Denmark seems very unlikely given Norway and Sweden are already here. And Songhai virtually overlaps Mali. Also pretty sure the Maori bumped out Kamehameha’s Hawai’i, and we’re surely not getting a Polynesian blob again.
(There’s always the hope of modded leaders from V Leaders Reborn for those though!)
But maybe something Celtic might still come, like Gaul instead of a blob. And maybe Italy with two city-state leaders instead of Venice.
It’s been a long time since we’ve seen the Hittites or the Sioux... and I feel like it may still be a long time, as much as we may want them back. (Nor are we getting the long sought-for Israel and Tibet. Here’s looking at you, Sukritact!)
For never before seen civs, there’s a small handful people keep rumbling about, and we might get one or two.
Vietnam, Burma, a SW NA tribe (Navajo/Apache), a SE NA tribe (Cherokee/Creek), Bulgaria, Romania (only if they want to do Dracula-the-civ), Ireland, Kilwa/Swahili (they’ve at least acknowledged their existence with a wonder), the Timurids/Gurkhani...
And I would really LOVE to see the Kushans or a Palmyra-themed Syria. Not that either is likely, but we could try to drum up support and meme them in like Georgia.
I’ve mentioned other ideas in the past but they don’t really seem feasible without everything else above coming first.
So that’s my analysis, for what it’s worth. I think I’ve covered all the missing prior civs (leaving out the obsolete Vikings, HRE, and the Native American blob) as well as the most likely new content.
Edit: And we mustn't forget the Goths.