[I've been thinking of a setting in which you have earth and a parallel world based on elemental cultures, and made up a new system of elements to go with that idea. Here is a hasty writing of that.]
"Ok, so there's another world--"
"About the size and shape of our world," she added.
"-on the other side of some mystical separator--"
"--which is is populated by magical beings--"
"--that are somehow related to elements in a vaguely classical model..." It didn't feel like a period, but he seemed to have run out of things to say. She waited for him to continue. Finally he said, "That cover it?"
"More or less."
"And this starts making sense when?"
"The classical elements correspond roughly to the four states of mater."
"Earth for solid, water for liquid, air for gas, fire for plasma and all that."
"Right, but there's an additional state of mater one needs to consider."
"Uh... quantum physics is so not my forte."
"Not one of those, simpler. But before we get to that imagine the four elements as the points of a square."
"Ok. Imagining it. This is accomplishing nothing."
"That's ok. Now we get to the other parts of the model you have to take into account. First, mater can be converted into energy and so energy is a state of mater."
"I'm calling bullshit."
"Not important. Energy, some call it 'light' is a fifth point. People usually visualize it by imagining it as above the four other elements and some say it's aether."
"Yay," he said. It was a flat, emotionless 'yay.'
"In theory-- never mind, getting ahead of myself. There's one more element we need to make the model work."
"Time's arrow. Entropy."
"Chaos and disorder and everything not nice."
"Something like that. Some people call it darkness because they're the ones who call energy 'light' and it sits opposite energy, usually visualized as sitting below the four."
"Energy hates entropy."
"It doesn't make sense, but you got it. Time for the other alliances. Mixing between the elements did two things. One was to warp them out of a perfect octahedron into something else.
"Specifically fire and water grew closer to energy while earth and air grew closer to entropy."
"One would expect fire-water and earth-air to be opposites."
"Yeah, and there's still some indications that that was once the case."
"In spite of being drawn in the same direction earth and air are still neutral at best and they tend toward laid-back hostilities. The same is true of fire and water. Air and water became enemies, fire and earth did as well. Air and fire retained their alliance, as did water and fire. In both cases because of their closeness, though water was just as close to air so... make if it what you will."
"I'm getting a headache trying to visualize this mess."
"You wouldn't be the first."
"You said mixing did two things."
"The other thing was creating another ... thingy. Not an element, just a thingy. A mixture of all eight elements. I call it nature. Life and death, hope and fear, motion and stillness, all of that crap. Nature has no alliances and no enemies. It doesn't take sides, it just is.
"Some say that in the beginning there were the eight elements that existed as worlds unto themselves, but as time went on they go closer and closer together, the creation of nature did nothing to stop that. In the end there was only one world across the shroud.
"All of the cultures were hodgepodges of all the elements, but each of the six major cultures had one that it, at the very least, strove toward. One where the members were mostly of a single element.
"Fire, water, earth and air all found homes on the surface. As one might expect air had a lot of flying going on, but there were still nesting spots and such.
"The energy culture lived exclusively at altitude. Never touching earth or water, seeing itself as above the others figuratively and literally.
"Entropy dropped into the darkness of caves and crevasses. Lurking under all the rest.
"Nature, of course, was everywhere. Inescapable, unavoidable, and totally neutral."
"You are aware that none of this makes sense, right?"
"Well, yeah," she said. "But that's the way it is."