I just feel like a lot of the combat is so static, and it could use some changes in the types of units available, like you could get more defensive units, or units that have speed but low armour, but not as fast as horsemen. The problem is that combat doesn't change between the ancient era and the atomic, with the exception of naval and airplanes, these are good additions, but ultimately, by and large the combat remains the same and this ought to change.
A return fire idea for ranged combat units could be a good one, this would also incentivise attacking melee units with ranged. Another would be having melee that are strong at defending against ranged attacks and a cheaper and faster and weaker ground melee unit, this would force people to change battle tactics.
My only reservation about all this is that the AI is already struggling hardcore, so maybe this would just make it worse?