So here are some means I use to make the revolutionary but overpowered MAGE class playable ( I mean seriously, unchecked it takes over nearly every other classes job, rivaling nearly everything except maybe the fighter, because at minus 1 he can do literally anything except three banned tags.). But here they are.
1. Have the player only cast spells that fit into what they specialize in. To compensate, you could let the player pick two fields of expertise such as both stars and winter at level one to balance it out. I also noticed gandalf is like this, specializing in fire/lights and healing. This I think is a simple fix to a big problem.
2. Is you want to have more costly magic, you could put an xp cost on each spell casting (I prefer to use this method instead of the vancian spells per day in the wizard and cleric class to make it more free form but limited. )Here is how a move would work. Roll 2d6 plus stat. 10+ means that spell only costs 1 xp. 7-9 means 2 xp. And 6- means you gain 1 xp as standard but bad stuff happens. Overall I think this makes play and magic use more organic and allows the mage to fill a very powerful flashy but balanced with other classes niche. My sister plays a winter and stars mage with both above methods in play and is happy with how the system works. How do you handle the mage if at all?