Ok, cool! That helps fuel some thoughts! And it nicely explains why all the tilesets must be black. I kept thinking, "but that eliminates any since of light! It's as if it is assumed that the tilesets are all underground and thus darkened."
Any side quests that have to do with unraveling mysteries of the past would I think be ideal. So, old rocket silos, or ancient human shopping malls which where completely buried and are in total decay (or is that still to near the surface?).
Are most of the light sources from organic or strange natural sources, or are there ways sunlight makes its way into these nether realms? I like the idea of these special locations where sunlight actually reaches into the realm of the knytts being an important destination, almost religious in connotation.
I guess I'm not giving quest/mini-quest suggestions.
Collecting artifacts touched by true-light (sun light), and collecting information about how to reach these locations would be cool.
I'll keep thinking.