I do hope that tile improvements change a lot during the game. With advancement into the late game a single farm should provide nearly enough for a city. So you should build lots of farms/mines in the beginning and later replace them with housing and districts. It would be realistic and interesting since districts and more special tile improvements would not be limited as much by tile space. City growth would not go through the roof by having 5 farms because it is limited by housing, amenities, whatever. It alway felt a bit strange that late game cities still needed that many basic tile improvements. A modern farm should easily replace 5 medieval ones. This might also work for mines since they got a lot more efficient in the last 3000 years... in Civ5 the improvements over time where there, but not enough imho. In this case districts are more a thing of build order and good timing (temporal specialization, preferences of the moment) instead of real specialization for the whole game.