The water temple environment originally started with a way more colorful and bright color palette but got pushed back to be more consistent with the rest of the environments.
The idea was to use a frozen water "curl/whorl" the player can use to travel up

For the Fallen City I basically just can say: too much Dark Souls... : D

The basic idea was adding windmill blades to the towers to take some weight from the massive structure. Inspiration for that was Nausicaä by Hayao Miyazaki.

general overview

My basic thought on fire was to have a natural structure connected with some temple bridge elements. The idea was to use a massive Obsidian kind of rock for the temple surrounded by magma lakes and a volcanic area.

Darkness - first mood

the final concept became way darker

ink sketch and silhouette

Earth should be represented like a mountanish area with forest elements to explore with the temple/cave on top.
For some reason I was thinking about Star Ocean: The Second Story all the time while designing.