By natural order, do mean Slayers having those aspect in order, like a Phoenix (Buffy) is called, then a Dragon (Kendra), then a Tiger (Faith), and the next one will be a Serpent? That would mean each aspect is equally common, which you contradicted in your descriptions, as you said the Phoenix was rarer than the others. If a calling order is what you mean, then I'd suggest abandoning it, and basing the aspects off of individual personalities. Which also means you could keep the seasonal correspondences as they are.