I think the age-transition was a missed opportunity, and as others have said, there could be 1-2 other ages I suppose.
Right now it feels very European centric, Fall of Rome | Discovery of the New World | Modern
We're definitely missing a feudal age, but then maybe that's specific to specific civs.
For me the ideal situation is:
- You pick the initial civ, and based on the crisis and your own choice of government could determine what/how a dark age develops.
Examples:
• A Collapse similar to the Bronze Age collapse or the Mayan collapse where cities become ruins, your people transition to settlers, cities become ruin tiles which provide cultural benefits, but can also become quarries. You settle new cities, but they are smaller in size but your population spreads.
• A Major Invasion, you select the civ and you get an army and get to conquer your old civ and some of the AI's old civs. But don't forget the Huns are right behind you!
* Maybe its just a decline into decadence like the Ottomans or Mongols had.
But these crisis IMHO should be an age unto itself, where the age ends when you birth your nation-state. And it should feel natural and based on what you've done/played.
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Then you have the exploration / renaissance period, I think one crisis could be either repelling the colonists or colonial independence where you then emerge as a new nation state in that sense.
The nation state should be the final civ you play and that should be custom, you get all your old city lists, and pick culture values based on your prior civs and leader.
My 2 cents

Granted, some of that would be a bit involved.