Great game, but limiting after Tier 4.
A school upgrade good improve the skills of workers further, and a university in general could research new technologies, new buildings, or likewise over time. That way, even big cities could develop.
Another thought is: People could get more skilled in their profession. A weaver doing the job for 20 years is likely to be more skilled than an apprentice. That would be super cool, IMO.
(Although one would need to keep track of all previous professions of every person)