Like military units can be merged in Corps in Civ6, there should be a possibility to join several cities into a Region. Yields are pooled, and excess food goes to a selected city, as well as production and culture (to push borders). Buildings' yield modifiers are shared by all cities in the region.
This way the number of objects under the player's control, and the number of production queues to observe can remain the same throughout the game, whats good for gameplay. Also, cities found later in the game will be more useful.
Incorporation into a region can be done by a Trader's action. More cities already in the Region, longer it takes. Or by a special Politician unit, for a sum of gold.