It is darkness and earth to be compatible with the theme. It wouldn't make sense with other quanta.
I just don't quite see the reason of so many new cards. Kael floods the card space with similar, complicated or both cards.
While the cavern is a really good choice for a dark/earth combo, i guess, it is impossible to find a suitable concept for each pairing.
I would suggest something else, each pairing could have a composite name:
submerged cavern - water earth
lava cavern - fire earth
shady cavern - darkness earth
...
Stuff that creates 4 quanta is OP, so i would stick to 2. For pillar upgrade it is enough to have the standard (works when played).