I used to play a game many many years ago called Genesia which had a fantastic research and development feature that would allow you, over time, to make new discoveries and inventions for your citizens. This would involve discoveries such as cures for illnesses, new tools and machinery like a wagon or plough. Eventually, gunpowder and firearms were even available. Something like this would make this game fantastic in my opinion.
I like this idea. Do you know the game 'Cultures'? Here, the professions (and the corresponding building) are based on each other. So, for example, to get a tailor, you first had to have a sheppard with some experience, and so on...