This being said, I do not see such a mechanic as very important. The classical games were very simple in their mechanic, so what was possible then, would, as the developer writes here, not be work with Ancient Cities, it’s too large, too complex, to simulate not only your own site, but also ten more of them. Anyway, I am perfectly fine with just ruling my main city, which will surely require hundreds of hours of game-play 
While @tschuschi is right in that games like Banished offer very little content after you’ve established a proper settlement, I believe that the developers will offer such depth, and such a width of choices, as to create much greater possibilities to keep playing. The already mentioned thread about political and cultural progression contains a lot of interesting stuff in that field. From the responses @UncasualGames has given in here though, and in other places, it is clear that they intend to recreate the very successful model of the classical city-building games:
- there are cities, tribes, villages and other sites out there, on the world map
- as your tribe and village grows, and learn more, they explore and discover more of the world map
- your tribe can interact with those sites on the world map, depending on distance, geography, knowledge, technology and other factors.
- all sites will be unique, some quite similar to you, some very foreign. Some friendly, some hostile.
- there will be trade, conflict, negotiations, diplomacy, migration, espionage and other mechanics, connecting your village to the other sites.
- because your tribe will be tiny and your people without knowledge in the first version of the game, set in the Neolithicum, we will see very little of the above mentioned points. But they will come in time, with expansions. Neither late, nor early, but precisely when the developers want it to…