I have three issues with the current upgrade / improvements system;
1/ the first is a basic UI change, where the first level of upgrade buttons will only be highlighted / clickable if there are improvements that are available. For example if you go into the blacksmith and have researched everything then the button is greyed out. Currently, when you are upgrading you have to click through each menu to check if you have got all upgrades. Not a huge issue but links into my 3rd suggestion.
2/ building improvements should be MUCH more expensive, especially levels 4-5 as they are far too easy to get.
3/ I think there should be more improvement levels, but smaller integrals of improvement and these improvements should only be unlocked at certain building levels. As opposed to the current situation where the player can spam all improvement very early in the game at lvl 1 buildings.
Veteran Archers - lvl 1 (can be purchased at lvl 1 archers barracks - half current upgrade power)
Veteran Archers - lvl 2 (can be purchased at lvl 3 archers barracks - same as current upgrade power)
Veteran Archers - lvl 3 (can be purchased at lvl 5 archers barracks - 50% better than current upgrade power)
These changes should extend the whole base development process, particularly if combined with needing resources to sustain armies as suggested here