Well, I think this is the fastest a game idea has ever crystallized for me! Hopefully I can transform it into an actual game.

A tip: don't be afraid to consider the theme in metaphorical terms! It doesn't have to be a game about digging or something. "Deeper and deeper" could go in so many less-literal directions...

Anyway, I suppose I should mention the tools I'm expecting to use (aside from the Unity):

Libraries: - probably FMOD and the Unity postprocess library

Visuals: - will likely keep them very simple, but might use Photoshop and/or Blender at some point

Audio/Music: - maybe Hookpad, probably Ableton, various VSTs

Misc: - just discovered the site for random color palettes, it's pretty nifty: https://farbvelo.elastiq.ch/