While at NorWesCon, I attended a panel on magic systems and how to get them right. While I was listening to the panel, it occurred to me that in my WIPs right now, I'm tinkering with four separate magic systems.
The Yowlerverse The Yowlerverse is probably the best-thought-out of the systems, and the most destructive of them. To understand it, though, you have to understand Aristotle's principle of mediocrity: Aristotle believed that, at its fundamental level, reality was the same everywhere. The laws of truth that applied to us applied likewise to the stars and even to the gods themselves. But mediocrity is only true at a fundamental level. It isn't true that the universe as a uniform 3C, except to a first approximation: within stars, and even here on Earth, the temperature is thankfully a bit higher.
In this reality, a few thousand light years from where you're sitting, a few thousand years ago, a star exploded. Everywhere within 30 light years of that supernova, its effects can be seen. Space has been swept clean. There's a "gap" in the Milky Way. (My learning of this led to the Great Kinnunga Gap in The Journal Entries, an area of space 2000 ly across that separates The Corridor from Sterling space.)
In the Yowlerverse, the universe takes the well-known Observer Phenomenon of quantum physics to an absurd degree: the sufficient will of a sufficiently conscious creature is enough to change material reality itself in drastic ways. This nicely solves The Drake Equation: there aren't any aliens because they quite effectively wreck their ecosystems the second they evolve to the point where they start to think conscious thoughts.
But Earth is "different." A few billion years ago something happened to create a supernova gap, but this "gap" is one where that fundamental truth of the universe is suppressed. Either something has been "swept out" of our space, or something has "blown through" this space, that has suppressed the will-to-power phenomenon, sufficient to allow conscious thought to emerge. Sometimes this suppression fails: famous periods in time when this happen include the early Greek mythological period (especially when Egypt still had pharaohs, as that created the Bastet), the mid-Roman Republic and Imperium, and a brief few months in late 1898. Oh, and the extinction of the dinosaurs? Totally a return-of-magic incident. Natural philosophers are terrified of the day this effect fails completely, because we're all doomed.
Moon, Sun, Dragons
In contrast to the Yowlerverse, Moon, Sun, Dragons (I discovered another series with a similar name, and am tempted to rename this The Carriers of Night) has a completely naturalistic universe. It just happens to seem magical. This is mostly a case of nanotech as Applied Phlebotinum, the seeds of the evil space dragon's destruction having traveled with the dragons from one star system to another, but the seeds require conscious direction and its hoped the humans can supply it. The problem is that the humans are all so limited in their understanding, this being the 16th century after all, that they apply the "magic" badly, and so we get the three classes of mage: swords, arrows, and seers. This isn't a limitation of the nanochine that's now infesting most of the Earth; it's a limitation of the imaginations of the humans themselves.
In the second Carriers of the Night series, Janae, humans have figured out a few more tricks, including a form of pseudo-prescience that's very useful on the battlefield, but still nothing outside the capabilities of ordinary human beings to cogitate about.
Aimee
The Aimee universe is all high magic, the kind of thing found in Harry Potter and Lynn Flewelling's book. It's mostly "whatever I can convince the reader I can get away with" sort of thing, with lots of silly ideas mostly ripped out of Rolemaster, which I always thought was a higher class of game than D&D.
The Reef
Technically, The Reef is a magic universe. It's just that humans can't do magic. But lots of other, very nasty, things, can. Since humans have no grip on the magical system of the universe, I don't have to explain it. I just have to understand it myself sufficient to convince the reader that Ms. Kensington and her crew can battle it and perhaps survive.