So one of the main issues I find with overbuilding is that almost every building is just a direct upgrade from the previous age. They mostly all do the same thing but with a slight increase in yields per age.
For example, a Library (outside of having 2 Great Work Slots) has +3 Science. An Observatory has +6. Meanwhile a Schoolhouse has +9. These buildings feel like a chore to upgrade since they don't do anything else except increase your yields.
So I propose that each building in each age should do something unique while also staying relevant the entire game. Here's an example:
- Library: +1 Science on Great Works.
- Academy: +1 Science on Quarters.
- Observatory: +2 Science on Mountain terrain.
- University: +1 Science on Specialists.
- Schoolhouse: +1 Science per Rural population.
- Laboratory: +1 Science per Urban population.
Every building would automatically have their yields increased per age. So if you stick with an Academy it'll give you Modern age yields in the Modern age. But essentially, these buildings do not just become obsolete.
I would suggest that you could only have 2 buildings of said type per settlement. So you can't just build all 6 types of Science buildings in the Modern era. If you want to change what building you have, you can replace it with a different one. This way you're not stuck with whatever buildings you built in the previous age. But it also means overbuilding isn't just a chore to get the most modern buildings for their yields, there's actually a benefit in doing so.
I figure almost all buildings in the game have some relevance in every era. A Library is still a thing in the Modern age. Hospitals, Banks, Markets, Barracks, etc. still are relevant to this day.
Some buildings might need to be renamed or redone. An Armorer might not make sense in the Modern era, but a Forge would. Same can be said about a Dungeon, perhaps it could be a Jail instead.
Models might need to be updated to be in line with each era, but over all, I think every building currently in the game can do something unique so that it always stays relevant every era.
