How do you decide that a magic system you’re working on is done?
When I feel like the stories I’m telling are working, I go with it. That doesn’t mean I have every little thing worked out – I’m a planner, so I have a lot of them, if I don’t have something worked out I like to sort of let people know in the book – here’s a hole that the characters have noticed, if that makes sense. But at some point you just have to write. So it’s a more by instinct thing.
I’ve created a couple magic systems and done worldbuilding, but whenever I think it’s done, I come up with another facet that I think would be interesting.
I say, if you want to tell stories, write the story and write down all the facets in a separate file. Then in a revision, go foreshadow that these things could exist, but don’t deal with them until you get to a future book. If you look at Mistborn, there’s hints about how Feruchemy works, but we don’t really deal with it in Mistborn since there’s so much already piled onto you.