I think it’s a fantastic idea. The open ocean is the physical barrier as it was for all people groups. Make sense to me. I doubt a mechanic like this exists , but for those concerned about the limitation, what if you can send out a colonist, explorer unit during the Antiquity age. They leave the map and based on a percentage of success, you have a starter city ready to be plopped down in an open space at the start of the exploration age. Basically you sent them out to explore and at the start of the new age you heard back from them if they survived. Not all antiquity civs would have this ability, only those well known for early exploration. That or one of the “victory” types gives you this opportunity if you meet a certain threshold.
It would also be fun to have random civs show up at the end of the antiquity age on your continent. Maybe that is one of the crisis. The sea people arrive and if not dealt with well enough they get the added benefit of immediately settling your continent at the start of the exploration age.
I also think a huge desert, jungle could function as a realistic physical barrier on pangea maps.