Tyranny had a fun system where you could create custom spells based around combining a Core Sigil with Expression, Accent, and Enhancement Sigils to modify the spell's behavior. So you could have a Fire core sigil, combined with an AoE expression, fast cooldown accent, and a bleed enhancement. Of course the spells have a cost attached to them so you couldn't have your mages casting ridiculously powerful spells on rotation until you ran out of magic, but you could pop it off once or twice then fall back on weaker, faster spells. Unfortunately like with most flexible magic systems like this, mage characters are overpowered as hell, as long as you have your party tank camping a chokepoint.