Note that once you unlock "Aegis robes" they'll prefer those to leather...
Right, before Aegis robes mages and light units are all in leather and all look like soldiers, after Aegis robes all are in robes and now all look like mages. It's possible to add a trait, for example "Initiate" and assign all robes and staffs to be equipable only if unit has this trait of Path of Mage and all other weapons and armor non-equippable. Then this trait could be added to the list unit design traits.
All units - mages will be upgraded with this trait. More robes with +1 Def +3 Def +5 Def and more staffs may be added to unit design system.
So all light soldiers will ramain to look like soldiers and mages will always look like mages. For soldiers having no more access to Aegis robes, Spectral Leather Armor with +5 Def may be added for unit design.
I only doubt in what bonus the trait Initiate should give. It should give some bonus which is valuable for mages and almost worthless to common soldiers. Perhaps penalty to physical attack may be good as addition, since mages deal non-physical damage.
Or may be Initiate should give just Healing? Every soldier doesn't need to be healer, players may prefer to give them Constitution, Prcision, ArmorProffeciency2 or other combat oriented traits, while future mages will have some magica benefit as Heal.
If you wanted to have robes be a separate upgrade path, you'd have to separate them out into a different item class (similar to how fire/ice staves were broken apart in LH vs. FE.) The armor auto-upgrade criteria in the UI only looks at Defense.
Interesting idea but it still allows to pack a mage in plate armor.
look more like Flemeth from dragon age allow them to continue to use leather.
Wasn't she wearing robes when she wasn't a dragon?