That idea has crossed my mind many times, but yeah, it's a very big and complicated project, and I'm not sure we would gain much by simulating this natural economical evolution that the player has little control over.I'm still of the opinion that we should have two build queues, one for the player (the state) and one for the AI (the free market) and that the production in the city should be split between them in a ratio based on civic selections. The people tend to build things for different reasons than a strategic player - thus a tavern would be one of the first buildings most cities would build from the free market side.
But as far as projects go... that's... big.
It would mostly act as a deepening of the civic system, but I believe there are more interesting ways to improve the civic part of the game.
An Idea I had on civics was to have a button within the civic screen that opens a policy screen, the different policies can be activated/deactivated without any anarchy time. Some are listed as mutually exclusive while others can be chosen freely independent of other policies, but the policies listed/available would depend on what combination of fundamental civics the player has and may also require techs.