There are civs in the "new world"

[stealth_nsk]

I'm not sure it's a great idea, either.

The player would need to be hammered with penalties or suffer loss of cities on era change for this to be feasible. If the player retains a great deal of land and cities conquered previously, that itself is a decisive advantage entering the new era. If the player does not, that itself is a problem for some gamers. Efforts made to feel... pointless.

IMO locking the player to a continent is not reasonable if anti-snowball is the motive. Snowball feasibility will be determined through other mechanics, chief amongst them being antiquity conquests and their retention. If I retain whatever I have after my successful warrior/archer rush, I'm already at 2x the scale of my competition and therefore untouchable. If I do not... well, kinda nihilistic.

Stellaris sorta grappled with the meta of early conquest. They added harsher and harsher penalties. Each time they did, for about 2 weeks players said early conquest is not meta, because 14 days is about what it took for the conquerors to prove them wrong. This may go similarly. Penalties, limits and restrictions seem like they'll make it more challenging, but, I'll find a way quickly, unless they're so extreme it's simply not possible to conquer at all, genuinely impossible.

The things we've seen so far are pretty good:

1. You can't transition to the next age until global threshold is reached for everyone. So, even if you are really ahead at tech, you have to spend techs collecting minor bonuses instead of rushing to some instant win later technology
2. On age progression all units are automatically upgraded to base units of this age. So even civs who struggle with strong army, catch up at this moment
3. Within the age you could enjoy some technological dominance with better units, but the difference won't be too big. Maximum difference is one step - warrior to swordsman or slinger to archer

So, nothing here feels harsh, but it also looks limiting enough to prevent owning enemies too fast

 

[Voidwalkin]

The things we've seen so far are pretty good:

1. You can't transition to the next age until global threshold is reached for everyone. So, even if you are really ahead at tech, you have to spend techs collecting minor bonuses instead of rushing to some instant win later technology
2. On age progression all units are automatically upgraded to base units of this age. So even civs who struggle with strong army, catch up at this moment
3. Within the age you could enjoy some technological dominance with better units, but the difference won't be too big. Maximum difference is one step - warrior to swordsman or slinger to archer

So, nothing here feels harsh, but it also looks limiting enough to prevent owning enemies too fast

In 6, the age in which the most productive conquests occur, ROI wise, is the ancient era. Most often, you are actually behind technologically during said conquests. The second most productive conquests, ROI wise, are done in the classical era(rolled into antiquity here). Here, you're on par technologically.

Tech parity doesn't meaningfully reduce conquest.

The pull-ahead happens in the medieval era, and because you're at great scale, the ai is non-competitive, categorically, everywhere. Scale is a decisive advantage on its own. Tech edge isn't strictly speaking necessary.

There are really 2 ways to reduce ancient conquests as the meta strategy I can foresee.
A: crippling happiness and distance from capital penalties, which would anger players. Even here, conquests would still be meta; you're just pushing to 8 cities instead of 17.
B: overwhelming ai production bonuses. I mean 500, 600%. This would slow you down. Conquest would still be meta. But it'd take longer.