So, I regularly play with "Dramatic Ages" enabled, because normal ages are boring and I like the dynamics of cities flipping a lot and the challenge of keeping a continuous Golden Age going. Having played this way so much, I can see a way forward for something I think a lot of folks have wanted in the game for some time now: New civs popping up as a result of revolution/schism/etc.
Basically, I'd love it if, when a Civ enters a dark age, there was the possibility (somewhat remote, but likely enough to occur several times in a game at least) for several of their cities to flip at once and become a New Civilization as a result. Scenarios in which I can see this happening, with some basis in history, would include:
Continental Divide: If you've got multiple cities founded on a continent different from that of your Capitol, when entering a dark age, there is the chance for them to flip as a group and become a new civilization. Examples: Post-colonial revolutions (i.e. America, Gran Colombia), but also the Byzantine Empire arising out of the fall of Rome.
Religious Schism: If you've got multiple cities following a separate religion from your founded or majority religion, when entering a dark age, there is the chance for them to flip as a group and become a new civilization. Examples: just tons and tons, but Pakistan and Ireland come to mind first. Byzantium could also arguably be an example of this.
Ideological Pressure: Ideally I'd like to see the return of Ideologies to Civ 7, and as long as I'm tossing around wishes, I'd like to see them done as in BERT, where they develop over time based on choices you make in civ development. In any case, the way this would work is that, if multiple cities in your empire are receiving some threhhold of loyalty/cultural pressure from a neighboring civ/civs with a differing ideology, when entering a dark age, there is a chance that they will flip to become a new civilization following the opposing ideology. Examples: Vietnam, Fall of the Iron Curtain.
Imperial Decline: Essentially what we see in the current game, where disloyalty simply snowballs as an empire loses control of territory. Example: Ottoman Empire during its "Sick Man of Europe" phase.
I'm sure there are other good ways that this could occur that I'm not thinking of right now, but the way I can see this working (based on Civ 6 mechanics because we have no idea what Civ 7's mechanics will look like) is that rebelling cities still enter a "free cities" stage (though preferably not as long of one as in Civ 6) wherein civilizations have a chance to fight for the free cities through warfare or loyalty pressure as now, but if free cities are exerting more influence on each other than existing civilizations are exerting upon them, that pressure - rather than creating just a region of endlessly hostile independent cities that you can't interact with through trade, envoys, or diplomacy - instead flip into a new major civilization entering the game. In this case, the free cities sharing borders would greatly increase the loyalty pressure between them.
Similarly, isolated free cities, after a long enough period of hostility, could transform into City States (à la Barbarian Clans mode), keeping their names but (assuming Civ 6 mechanics for City States) gaining the attributes of a CS not currently in the game.
The Continental/Religious/Ideological situations I'm referring to above would basically determine which cities are lost at the start of a dark age - like if a civ is going to lose at least two cities, and one of these sets of circumstances applies, then those specific cities are the ones lost (rather than just whichever cities have the least loyalty pressure, which I think is how it works now?) and a message appears saying "You have entered a dark age, and your cities following Eastern Orthodoxy have declared their independence" or "Ethiopia has entered a dark age, and their colonies on the continent of Lemuria have declared their independence," or similar.
Much like civs have preferred religions that they'll go for in Civ 6 based on history, there could be a short-list of "related civs" for each civilization that are more likely to arise from these situations, so that it's not entirely random (these could be based on history where applicable or just geography where not.)
There's a lot that I'd love to see (and don't expect to see) in Civ 7, but the kind of dynamics of world history that could result from this kind of system would probably be my number one choice if I could choose anything.
