I thought I saw this somewhere in the forums before, but searching for "addons" and "building addons" turned up no recognizable results, so I'll just post what I thought up.
Ok, we have building plots, we have research, we have upgrades, we spend influence to unlock permanent improvements for everything in our home city and our armies.
But one particular thing we lack, is variety. What makes me different from the other hundreds of rollingplains humans, or desert orcs, aside from the fact that some others have played longer and have more developed.
Where as in general we could use a little more specialization to be more unique, heres a nice little way to start, suppose for each structure (much like our wall towers) we can add a building addon (or multiple) to act as a unique buff provided by that specific building, these bonus should also accumilate on each other to provide a benefit to creating duplicate structures. Here are some examples for human race. I use (Unit Type) to represent this could be anything from swordsmen to knights, depending on what balance testing might feel appropriate.
(Unit Type) Guild: (Unit Type) mercenaries cost 10% cheaper
Heroic (Unit Type) Statue: (Unit Type) have 5% increased health (or morale in future update)
Veteran Instructors: (Unit Type) gains experience 25% faster.
Medical Supplies: (Unit Type) is 40% cheaper to heal at towns and have a higher heal rate.
Drill Yard: (Unit Type) drains stamina 30% slower while sprinting.
Economy structures could have certain economic or utility addons and the sorts.
The addon's don't have to add any cosmetics at first, they could just be statistical buffs affecting the army as a whole. this way players can have a home city that feels more unique than simply the weapons mounted on the towers and the traps. Because eventually everyone is going to have every upgrade and research unlocked but this way we can still be unique in our own sense.