[Edi]

Those are the ones I would do if there were to be an Elemental Royal corrupted by some path of sorcery in each element. They are the most natural fit.

One can also argue that air and water are less prone to be corrupted than Earth (stationary, unmoving etc) or fire (corrupted through the fuel it burns, the fuel having more in common with earth). Air and water are both fluid, capricious and wild, bot as easily tamed as fire and earth.

However, this is CBM stuff, since I doubt that it would get added to Dominions even if the game were being as actively patched as in the past. The Father Illearth and King of Banefires backstories make for great drama, which isn't as readily available from the Celestial/Life angle, those being more "good" (as much as that word applies here).