You would probably have fun building a hard magic system. Hard magic being a system that has strict rules for how things work. Soft magic being Gandalf style where how it works and how it's limited is unknown.
Although "unknown magic" and "ancient powers" are convenient, it bothers me to think there's no consistent system underneath it.
In the setting of the world, people in the world can view it as unknown, but the author is the god and should have a decent idea of how everything works.