I'll just throw this out there, it's something that sort of popped into my head while playing the other day. I had been fooling around with the sprawling trait (gave it to every leader to see how it would work), and thought that if you can make different types of cities with unique build lists, IE normal and settlement, would it be possible to expand on that? Then my mind quickly jumped to forts. Say you replace the fort upgrade with a new type of "city" that's built using a new type of settler. This city would be locked at 1 pop like a settlement, but it would be able to build unique upgrades that could be unlocked with technology. So this new fort would start with like 20% defense, and right away you can start building a wall upgrade that adds another 10-20% defense. Future upgrades could include further wall upgrades, maybe a medic station to increase healing, and even fixed defensive systems like turrets or ballistas that can't move but can fire like normal siege weapons from the fort.
I'm not sure if that's possible at all, but I thought it was cool when I thought about it so I figured I should post it, especially since this thread talks about making forts more useful or interactive.