#1:more units for warefare, pikemen, heavy cavalry, crossbows, muskets and cannons would add lots of depths to the game's warfare

#2. More resources, things like gold and gems that could be worth lots of money but that can only be acessed by building large mines deep into the earth, other valuables like coal (same function as charcoal but can be mined), copper, gunpowder, and other valuables

#3. Huge islands that could have multiple ai on them with the player, a kingdoms population correlates to how much land they can claim

#4, ive seen others talk about this before, higher and lower ground to show natural geography

feel free to post any other suggestions below