A Bit More Control For The Player [Abilities/Stats]
Give the player a bit of customization over their skills. If you don’t want to do skill trees an option would be to give the player points on their level ups and distribute them in their skills as they please allowing them to feel like they are the ones creating their character and their mage is different from the other mage in the party. Keep abilities level locked as you progress you unlock more options.
Player Primary Stats
Going back to some D&D maybe every couple of levels say 10 or so allow the player to allocate a few points to increase their primary stats such as strength constitution and the likes. This will also give the player a sense of uniqueness