What could add more “things to do” is for example adding more resources, which you need to make more high-level products, and nclude more exothic and/or imported-only products to maintain tier 5 buildings (an example of what I mean is here). Also add more upgrades for some buildings like storages, some industries, etc. All this, for the mid-high level.
For high-level, I think the only way to extend the playability is to reach a point where your city is enough powerful to start exploring nearby regions to make colonies.
The best way to do so could be the need to establish a more defined form of government. A good tier 5 city should introduce high-level services like high schools and/or a university, then a place where justice is run, so a courthouse, a “tribunal” or something. And, most important and above everything, a government palace (no need to specify what form of government is - also an equivalent of this might be a real upgrade to the “town center” once reached the level 5, renaming it and giving it a new shape as a true grand government palace) which unlocks all these new high-service buildings. Also, a public bath house is needed, which uses wood to heat the water and keeps higher the health and especially the happiness. And a public street lighting (I suggested this times ago and also a possible way to implement it here).
Once you have all of this, an option to explore another map should be eventually given, which is “connected” to the city (meaning not physically, but as a “child map”, with which you can exchange resources and citizens, and which is generated in a way to include some special resources not available - or which were available in low quantity - in the main map).
I think this way you can literally double (if not more) the longevity of this game.