This might be part of the solution to the real City State problem in Civ, in that City State Civilizations are very badly handled in the game, and that includes some very important civilizations in history. Aside from obvious candidates like the Phoenician city states of Ancient/Classical Eras, Greek city states of Classical Era and the Italian city states of the Renaissance, I suggest that many early 'states' consisted effectively of One City plus its conquests. It was only with the 'invention' of the Empire style of satrapies under Cyrus the Great of Persia, later perfected with the Roman Citizenship of Rome, that you really had the kind of centralized Overall Control and Coordination that we think of as 'normal' in Civ.
So, as a suggestion, combine the Province concept with the Puppet City concept.
Every new city conquered is automatically a 'Puppet' as in Civ V: you get the benefit of all or some of its Science, Amenity and Food production, but you cannot actually specify Construction of anything.
Every new city founded, if it is more than X tiles away (actual distance would vary with map size, Technology researched, and could possibly even be linked to Trade Route Length - communication by sea allowing the city to be further away but still under Central Influence - would also be a Puppet.
Any two or more cities with the same Culture Background and in proximity to each other could form a Province. That is, two cities you founded on the same island, for example, might turn all or most of the island into a Province, and two cities conquered from the same enemy Civilization which started as founded by them would form a Province.
Provinces could be Specialized in that you could either tell them what to Build OR simply specify that the Province is 'specialized' as a Food Producer, in which case they will build Granaries, Builders, and mostly Food Producing Improvements.
Provinces that Specialize in Defense AND came from a different Civ or are in an entirely different terrain/climate might even be able to Build Unique Units that your 'normal' Civ can't - your 'Scythian' province cranking out Horse Archers, for instance.
The size of a Province, again, would change with travel and communication Technology. By the Industrial/Modern Era, a Province could be pretty large, but you would have the choice of 'dividing' the provincial areas into smaller States, Provinces, Strapies, etc to better specialize them as needed. What you could not do, realistically, is re-arrange a Province after it has been set up for a while: people become attached and identify as Wurttemburgers or Texans and will not be happy if they are suddenly redesignated as something else by the central government.