As an imagination, if aoe2 units have stamina, it would be potentially fun. For example a horse running across map would use up stamina and slow down, and then need some rest to start running again. Different stamina for different units, walking or running or riding horses/camels. There should be techs and bonuses related to stamina too. Just a fun idea.
For me it would be fun if units insulted each other during fights but it won’t happen.