Two ideas that would really add a lot to the game.

Old districts

First, it would be so cool if we had the ability to not upgrade districts from one era to the next.

It would allow us to create even more realistic cities with old districts and new districts.

It could be just a visual thing, or, it could be tied to some game mechanic where the old districts would not upgrade their yields but would instead gain influence per era.

It could be done through a buildable district similar to the nature preserve.

Two examples below, first a classical district in a modern city, then a medieval city.

Visual story

The second idea would be awesome for the end game recap and charts.

It would be a short slideshow of the evolution of our oldest (or capital) city through the game.

It could be simply made of automated screenshots taken at the end of each era (when players gain their last era star).

Yeah, I'd like if the "borough models" changed slowly over time to give a more cultural mosaic, mostly happening when new boroughs are built or when upgrades (like industrial, pollution generating infrastructure) comes online that changes their appearance anyways.  Maybe quarters next to influence-producing quarters would be slower to change culture.