Here's Stuart Jaffee from Magical worlds on magic systems as a way to develop characters.
"Think about how you build a character when you are doing the ground work before you write the tale. Part of it is simply sitting back and letting your imagination play. And part of it is asking questions. How does this character feel about hard work or religion or putting herself in dangerous situations? What does this character fear? What’s this character’s favorite food or color or movie? In fact, the majority of character building can be summed up as asking yourself questions.
Well, I’m sure you see where this is going — the same can be said for building a magic system. The initial part is just letting your imagination go. Let it play a game of “What if?” and see what happens. But the second part is the crucial “asking questions” phase. What does this magic look like? Can anybody use it? What is the cost of using it? What are its limits? Is it something people and/or animals are born with or is it something developed over time? Do you have to go to school for it? What are people’s attitudes towards the magic and those who wield it? The more questions you ask, the better you will understand how the magic works."