Revolts/Rebellions should definitely be part of game for any player with significant territory. However, they shouldn’t be “city flipping” instead they should be like an invasion from within.Colonies also just weren't much of a thing throughout the world, or have been held on to. Russian colonies in Siberia were mostly established after the rough beginning of the Modern Age and they still have them today. Same thing mostly applies to China, even if the PRC had to reconquer the west after the civil war. During the age transition the Ottomans were *gaining* territory (not much though) across the Mediterranean. I'm not sure it makes sense as a mechanic (especially as some players will simply not settle other continents as a result) compared to the more widespread early modern revolutions that embody the industrial transitions of the 19th century.
A “revel activity” meter you can see goes up. When it’s high enough, enemy units start generating next to a rebelling settlement and they try to liberate it.
You can keep troops in the area to hunt them down, you can do things to boost happiness in the area to reduce the rebel activity. or you could grant the settlement independence/autonomy becoming an Independent People that you have a very good relationship with
If they take the city instead, they become an Independent People that are hostile to you.
That IP (or a group of them) could then become a new civ in the next age.
That said I really want the next playthrough to be playing through a Crisis, to see how this works.