For The Magicians in particular you may want to expand (greatly) on the importance of rotes. Getting access to spells was super important thematically, and often the real difference between street level casters and your more formally educated variants.
Giving some teeth to improvised magic (or rather, its consequences) and maybe adjusting it so it's easier to fuck up a spell might also give you a way to keep the game low level, if you wanted to lend some rope to your players to hang themselves with.