Boris, as (almost) always, has a strong point.
A more fluid civ-changing system that reflect evolving game conditions, rather than a strict straitjacket age system, would likely be a good idea.
I've been toying with an idea of replacing ages with revolutionary technologies (/research, could also include some civis in there).
1. They are tech chokepoint gating access to a lot of technology behind them. Past a certain point, you cannot progress (at least in certain directions) without researching them.
2. They cause a lot of prior civic, wonders, buildings, etc to become obsolete - a classic mechanic -, thereby (temporarily) greatly reducing the advantages of empires that are ahead in the game.
3. Once any civ has researched them, their research cost is drastically reduced, and their prerequisites are waived, making it much easier for other civs to jump ahead in the tech tree and catch up
Discovering a revolutionary technology could tie in to civ switching by causing some of your civ bonuses to become obsolete and opening the option of changing civilizations (but not forcing you to do so, I suppose). Some civ could even have multiple bonuses that are obsoleted at different revolutionary technologies.
This would also mean that different civ could be unlocked or have their bonuses obsoleted at different times without needing fifty two billion ages.
I like this idea. I'm in the Pro Civ Switch camp I more or less look forward to age transitions, but they certainly can evolve or work better .
Gating the techs in each era or age, will allow for some catch-up, you could pair this with the crises to show that it is a pain to get past these chokepoints, then bunch up a bunch of the toolsets for civ switching right after that point.
2 or 3 extremely high cost techs and civics that must be researched before moving on, who's cost dramatically reduces once it's first is discovered.
What might make a bunch of people happy could be staying with your civ flavor (Names, architecture, unit styles) but adopting the traits of a current age or current tech civ.
"You can now adopt the Norman toolset.. it will be usable once you research Castles"
Upon reaching Castles, You are prompted do you want to:
A) Skip Normans wait for another toolset
B) Take Normans and switch to their theme set
C) Take Normans and keep your current theme(Names/Art) matching that era.
This would allow toolsets to unlock at ANY point within the half dozen or so traditional eras, you can switch once per age, or however often you want to allow for it.
If you started the game as the Normans than you get a "classical roman" art set when you start the game, but are prompted for a starting ancient toolset.
Every toolset has an obsolesce point where you can no longer build them, but like the unique quarters, continue to provide a bonus.
This might allow for more diversity of civs added, as you can put their bonuses really early or late in an age.
One more feature I miss and correct me if this was a mod, but late stage CIV 4 had population migration and each city showed a breakdown of your population by civ, similar to how religion was in 5 and 6. This fed into happiness system I believe.
I'd take that a step further and allow uniques to be built or traditions used, if you met a certain percentage thresholds.