The system of elements--a combination of literal elemental power and schools of magic--was probably the first thing I ever tried to design for the Demonsword Project, and I have never really settled on what they ought to be and why. I think I have found a solution that I like--but I've thought that before. I would warmly welcome any comments on the matter.
In my notes (and don't worry, I'll explain this), this is the 3-8 system; it's very close to the 5+2 system I have been using for the last good while, which evolved from 6+1 before it. Indeed, 3-8 could also be seen as 6+2, and unfortunately that involves adding a new element; however, this one makes more sense than 6+1 or 5+2. Of course, it's come a long way from the 1+2+6 elements I started with, so there's that.
You see, in general, I divide elements into "standard" and "special"; the 6+1 was "Six standard elements plus Void", and the 5+2 was "Five standard elements plus Wild and Void". The void element, in all cases, is a zero-element associated with clarity, perception, and divination magic; Wild magic in the latter case is related to life and involves combining all other elements together. In all cases, each element is many things; a type of energy, a part of the natural world, and a school of magic. They are each related, but they are not all three exactly the same; fire-elemental "energy" and fire in the natural world are related but not the same, and the Fire school of magic is not strictly the same as fire energy, either.
The 3-8 system isn't written with a plus because the distinction between special and normal elements isn't quite the same. There are three fundamental types of energy, or if you prefer, major schools of magic: internal, external, and barrier. The eight (2³) elements are the zero element, the all element, three pure elements, and three mixtures--all the possible combinations.
In the fictional canon, then, a novice elementalist would gain skill in manipulating the Three Energies, and in doing so begin to both control the elements, and learn magic. As I'd originally intended, this practice with energy is much more like psionics than Vancian magic, and canonically is much more touchy-feely sense-and-manipulate magic. It also puts more natural boundaries on elementalism, but I suppose that has more to do with the flavor of the canon than anything like game mechanics.
For now, the breakdown of the elements looks like this:
Zero Element: Void
Pure Elements: Fire (Outer), Earth (Barrier), and Wild (Inner)
Mixed Elements: Wind (Outer+Barrier), Water (Inner+Barrier), and ??? (Inner+Outer)*
All Element: Thunder*
The two asterisks there are the name and nature of I+O Magic, and whether I really want Thunder to be the "ultimate" element. It feels right, based on the properties of Thunder magic, but it also seems kind of athematic; I liked the idea of life magic requiring all elements in balance. To the first issue, though I+O magic seems like an element of transfer, one that connects worlds, and so I was thinking possibly Astral as the name.
Although a full discussion of the metaphysical precursors and implications of the 3-8 system is well beyond my ability to get into tonight, it suffices to say that "outer energy" relates to manipulating and sensing what goes on outside the body, in the physical or astral realm, and "inner energy" relates to manipulating and sensing what goes on inside the spirit, soul, mind, and similar realms. Barrier energy relates to the both the physical form of things--energy that has bound matter into a particular shape--and the boundary between inner and outer realms.
Using that as a brief jumping off space, Fire simply dumps energy into the world, destroying things, Wild energy creates an energy surge within things, Earth magic manipulates physical forms, Wind can form a physical barrier or be used to physically move or stop things (it draws energy from the environment and uses that, instead of the caster's magic, to strengthen the barrier), Water protects, enchants, curses, or binds anything that can contain Inner energy (drawing energy from that thing to do so, especially bindings and curses), Astral transfers energy safely between inner and outer domains (for example to strengthen you from outside energy), and Thunder is a strong magic that ties all domains together into a cohesive, energetic structure; it is useful in all of the lesser domains, destroying, surging energy, manipulating structures, creating barriers, binding and enchanting, and transferring energy. As previously, of course, Void is a magic where you cannot do anything for the duration, but in exchange you gain a clarity of perception, especially into the workings of ongoing magic.
Again, the metaphysical implications of this on a world are an interesting topic of study; for me, however, the main takeaway is that the magic all works together in much the same way as I'd been expecting. Nothing about the new system changes the way demonswords, wardwraiths, wardwalking, hypermentalism, proto and protoviruses, or really anything else works, and it does not fail to work together with the Spare Change Project's Chaos Key. So, I'm happy.