Brandon Sanderson, the author of Mistborn, The Stormlight Archive, etc. presents three points of consideration for fantasy authors.
First Law
Solving a problem through magic depends on how well the readers understand the magic.
You cannot simply have vague magical powers and then use a wizard to enchant problems away. That would amount to deus ex machina, an unsatisfactory read at best, a betrayal of the readers at worst.
Second Law
Limitations are more important than powers.
While magical powers are interesting, they cannot alone make a story. Even if the first law is satisfied and the readers understand exactly how the magic works, if the powers can trivially solve problems, that is not satisfying. The failings of magic are what create the core conflicts, forcing characters to find nonmagical solutions or to use their limited magic in creative ways. And that is how the story becomes engaging.
Third Law
Expand on the existing before adding something new.
Having too many kinds of magic overwhelms the readers. It gets in the way of both the laws above. If the reader had to split his attention on too many kinds of magic, he won’t understand all of them well enough for the author to deliver satisfying magic-based solutions. The limitations of the system won’t be clear; and that would affect the generation of conflicts and resolutions.
Reference
- Brandon Sanderson, What Are Sanderson’s Laws of Magic?