suggestions: in the same way that hovels appear to being upgraded automatically to shanties as i level up, upgrade my materials/research production buildings (both existing and available) when i level up too (with the same maint cost ideally). that way you give people an additional incentive to reach levels, AND you reduce how many i have to build in the later game. if it was up to me i'd use the % bonus buildings for this and move all the % bonuses to the level up bonuses instead. this would give much more purpose to the level up idea (and meaning; currently i don't see the point in level up bonuses because everything they do could be better handled by buildings)
Honestly, I don't like the auto-upgrade mechanic very much. In some ways it makes sense that I researched to shanties and now I may want all of my hovels to become shanties. Yet, at the same time, I now can not even build a hovel in a level 2 city for any reason, and as a player, I lose any reason to go back and manage my cities housing, since it completely done for me. With respect to hovels to shanties, it makes sense that your hovels upgrade simply to shanties as they are just better houses which have the similar maintenance values. However, with respect to upgrade resource buildings, I think it would be more interesting strategically if these buildings were not simply the same building, just one for level 1 and one for level2 with an upgraded production. It would be far more interesting if your level 2 study cost like 2x the people and maintenance for 1.9x income. The idea would be to allow a higher level city to make better use of its tiles, but not simply make a level 2 building which was equivalent to 2 level 1 buildings. Its all about giving players choice.
suggestions: reduce or remove gold maint from some buildings. i really don't see why i need to pay to maintain a wall. for me the cost in that should come from the research and construction itself. same goes for unique wonders and the such like. i should only have to pay to maintain buildings that produce stuff like research, materials, training facilities etc. instead maybe add citizen requirements for some of the buildings that don't have them now, or increase requirements for those that do.
While this sounds good on paper, walls take more time and energy to maintain than one might think. Weather and wildlife can wreak havoc on walls and other buildings, causing maintenance to be a real concern. As for wonders, these buildings still have workers that need to be paid, and these wages are part of the abstraction of maintenance.Yet, beyond the reality checks, maintenance serves as a limiter for balancing player choice. Without them, there would be no reason not to simply build every building in every city.
remove the requirement to build an actual building when you build a city wall. what does this achieve other than get people nearer to the tile limit?
Here I think you have answered your own question. Building defenses should not be "free" for a city. It should most definitely force a choice upon the player to either build defenses or more civilian structures.