[Refplace]

It could be as simple as every mage has their own quirk or in extreme cases even a disadvantage reflecting how they do magic.
Some have to sing, others rhyme, some use a focus object of some kind, others can only cast at a certain time, etc.
A Noun/Verb system could also be distinctive, especially if there were different ways of preparing and using the symbols and a large lexicon where most mages only know a few.
I have a Ritual Magic system where the colleges are replaced by "Affinities". Its mostly taught as cohesive whole but individuals can invent their own.

The biggest decision is do you want highly varied magic or is it mostly a difference of personality in application?