While working on my Aethoth worldbuild (secondary world fantasy), I’ve recently been considering, and decided to implement, the idea of evolving magic.
We commonly see a variety of magic systems at play in a setting, ex. Tolkien (wizard magic v. elf magic v. dwarf magic), Brust (sorcery v. witchcraft), or Pratchett (wizard v. witch). We also regularly see a decline of magic, ex. Tolkien (the fading of elf magic and withdraw of the wizards) or Pratchett (sourcery -> wizardry). The best I’ve seen of what I’m getting at with evolved magic, though, is Ilona Andrews’s Kate Daniels series, in which magic is constantly changing and adapting to a post-apocalyptic Earth. To some degree, the Dragonlance (5th Age?) approach might be akin to what I’m thinking as well, but I’m not really familiar with it.
For Aethoth, I’m thinking that an old race (the aiser) used a rune-based magic millennia ago. This was later adapted by one of the older human nations into sigils—stylized forms of the runes—that no longer connected with the aiser language. Then in the modern world there are three magics—sorcery (blood and/or spirit magic), witchcraft (ritual and/or focus), and wizardry (visualization and willpower)—that all come from the sigil-rune roots adapted to a new era and changes in the nature of magic.
This may tie into the concept of magic as a living entity. As such, magic would adapt over time, changing to better survive within its environment. The old throwbacks still exist and work, perhaps even thrive, in certain niches areas (ex. sharks and crocodiles), but the new versions have better evolved for other, more common, niches too and are therefore much more common.