I have a few ideas.
Morale - it changes depending on the victories and number of retreats of the unit and may affect the damage or luck the unit will have depending on the cause.
Custom names for units - this allows the player to customize the name of the unit he owns like 61st infantry regiment and Legio IX.
Custom themes - this allows players to choose a theme like; ancient times, where the artillery unit turns into a catapult unit and ww2 theme that changes the cavalry unit into a tank unit. The tank unit is a bit stronger than the normal infantry unit but has higher luck or damage when supported by an infantry unit