I had always wanted crystal as an element until psionics was canonized, after which I have been staunchly opposed to the idea of so many elements. The idea of having psionics—literal mental processes—as something wieldable by a Toa as an elemental power alongside actual physical things like stone and fire and air and iron struck me as entirely absurd, and shortly after its canonization there was a push for making inertia an element, which thankfully was never made real. I get plasma, and even gravity to an extent, although in hindsight gravity makes only a tiny bit more sense than psionics.