Wow, I love this game. This is one of the best I've played so far, I looove that you made something visually compelling with only one sprite.
I wasn't entirely sold on the "dark level" idea until I realized that you got to see what your past lives did - that's super cool and adds a whole new layer to the game! There was a genuine suspense and curiosity in exploring the dark level, and I think you're on to something here. Maybe after the jam you hide extra "impulse" pickups around the levels, providing more exploration and a chance to play with that mechanic more.
The player character felt a little clumsy, and I'm not sure it was registering all my jumps, so that's a spot that you could focus on re: polish!
Check out our game Einnsámr too :) https://itch.io/jam/gmtk-2019/rate/463168
P.S. - I'm a game audio professional, and I do declare that those are some phat beats if I've ever heard 'em!