If I may dream:
Walls that protect against projectiles if behind a wall, except for "artillery"
Abitlity to build a bridge (Settlers 5 had that) - this could preconfigured sites on the map where the path crossing a river or so is initially blocked (like a closed locked gate) but "somehow" indicating a bridge could be built here. Building a bridge basically means creating a building animation and when finished opening the path ("unlocking the gate") thus allowing crossing the bridge - similar to opening a gate in a wall or palisade. The mechanics must however be inverted: default closed for everyone. Only if the bridge exists the path is open - but for everyone. Interesting gameplay could be that you could destroy bridges as well thus closing the path again.
Ability to build roads that have a speed aura for everyone following them.
Adding water as a kind of resource - i.e. building a field in the desert does not make much sense unless you are close to an oasis (open water body) or build a well or fountain to get water from underground (romans of course build aquaeducts ). Crop yield could be low if no water with a limited lifetime of the field, and high if water is provided by a nearby source. For existing maps, the well house is the only possible source of water. For new maps, an area around water bodies (oasis, rivers, lakes, ponds...) could be defined that allows farming. On desert maps this could force players to fight for water access.