Salts are the products of acids, but that's pedantic.
Heh, fair.
I've started moving towards more para-elementals as I feel the classical elements are over-done for me. But, I absolutely agree that they need some jazz. 2000 years of tradition and lore is hard to replace.
I've gone with:
Mist - Air/Water (Infiltration, Moist, Cold, Ephemeral)
Clay - Water/Earth (Mutable, Foison, Fertile, Potential)
Magma - Earth/Fire (Sporadic, Explosive, Metal, Hot)
Radiance - Fire/Air (Rapid, Warm, Color, Expansive)
Salt - Water/Fire (Dross, Preservation, Lingering, Dry)
Dust - Earth/Air (Entropic, Spreading, Abrasive, Clinging)
It is easy to contrast hot dry fire versus cold wet water. Where rain comes from, so does does snow.
But both earth and water are "cold". This is why I like associating the Cold damage type with both elemental Water and Earth.
With regard to atmospheric snow storms, it is the watery aspect that produces the Cold, not the airy aspect.
The airy aspect, I associate with luminous daylight, and Radiant sunlight. While mundane fire is hot Air, it is especially the sunlight that is the element Fire (the state of matter). The entire sky glows with daylight. Daylight and sunlight are often distinct concepts, since twilight has a luminous sky apparently without the sun. I associate the Radiant-Fire damage types with both Air and Fire.
Regarding "clay" and "dust" as paraelements, heh, it is difficult for me to find these "exciting", or even meaningfully distinct from each other, since clay actually is dust particles.
For this reason too, I prefer the Water-Earth paraelemental be Cold (and darkness) itself.
Magma ≈ metal. I also associate the Fire-Earth combo with "forging" metal items. So the correlation of metallurgy here makes some sense. This would also relate to the alchemical aspects of Earth generally. For the sake of picking a D&D damage type, I end up with Acid making most sense for Fire-Earth. Relatedly salt is a "fiery" tasting "crystal", whence salt is also Fire-Earth. But I also like the connotation of Fire-Earth being "metal".
Note, water can also deal Bludgeoning damage, as well as pierce and slash thru rock erosion. So the Weapon damage types (Bludgeon, Pierce, Slash) associate with both Water-Earth. It is even possible to make high pressure water jet into a kind of blade that can cut thru metal.
For the Air-Earth paraelemental, this can be the Tree itself as an element, whose branches and roots unite sky and land.