GS already seems like the heyday of the “weird” civs, at least in terms of play style quirks.
I don't think they've remotely gone full weird yet.
We don't have a civ designed around city-state puppeteering yet (just levying).
We don't have a civ designed purely around religious conversion.
We don't have a civ designed around espionage or cultural infiltration.
We don't have a civ designed around barbarian politics.
We don't really have any civs built around an "ally" system yet, and I could see a whole expack's worth of civs developed with passive group-hug-ish abilities.
We could also get an entire expack built around non-military unique units.
Not to mention some design space for weird economic playstyles still exists, especially if they expand into corporations/markets.
I think the devs have finally realized that they can go in a lot of bizarre directions if they want to. I mean all of Gathering Storm was based around terrain bonuses, maluses, and literally fighting against the map itself. That is wholly antithetical to the traditional Civ notion that civs are designed in a vacuum to all have a unique or two geared purely toward self-advancement. We are, for the first time in the franchise's history, getting (more) civs (than Venice and the Huns) that inherently care about
everything else going on around them, and in that respect I think the devs haven't come close to revealing their hand.
(also, it would be a complete waste of interface design to give the hall of fame a scroll feature only to include one measly extra row...)
If there is a 3xp, I think we'll get Vietnam. For some reason, Vietnam is a highly popular civ (Americans wanting to review the Vietnam War?

). I remember old threads in which Hungary, Canada, Kongo, Vietnam and Maori were highly requested civs, and now only Vietnam is not in the game yet.
I think that line of reasoning is in part informed by a presumption that the third expack will be the last eight civs we ever get in the game.
While I do think Vietnam is quite likely, I think it will be one of the last civs added, whenever that is. So if we get a fourth expack or DLC, I in no way expect Vietnam to be in expack 3, when the devs can keep holding it hostage and release less popular civs that they still want to include. Like, say, Burma.