does anyone know any other options that could add on to water, rock, air and fire that would be great both from old myths and more modern tales?
My choice would be Void.
My default cosmology for D&D, more fully described in
this post, follows these five broad categories in descending order:
Fire - The Positive Energy Plane, WIS, (Con), radiant energy/radiance, light, spirit, spiritual fire, this is basically that from which everything else (except Void) is made.
Air - The Astral and Outer Planes, INT, (Dex), thought, within this region, the Upper Planes have an affinity with Fire, and the Lower Planes have an affinity with Void.
Water - The Ethereal and Elemental Planes, the Feywild, and the Shadowfell, CHA, (Str), emotion/sensation, the Feywild has an affinity with Fire, and the Shadowfell with Void, spirits of the four physical elements (fire, air, water, and earth) mainly reside in the Elemental Planes.
Earth - The Material Plane, STR, DEX, CON, (bodies of) the physical elements, physical reality.
Void - The Negative Energy Plane, necrotic/necromantic energy, darkness.