I don't know how difficult this would be to implement in LOVE, but have you considered a dynamic soundtrack? Like, the music changes slightly as the difficulty increases? The soundtrack is really cool, by the way!
I will have to think about this idea as I would need to pretty much redo the soundtrack with a bunch of separated layers, but assuming I have this done at some point in time, it wouldn't be too too hard to make the BGM dynamic, as all there would be left to do would be most likely along the lines of changing the volume of certain layers depending on the level the game is at.
I guess the biggest problem is how to adapt it to both speed level and piece set. I managed to get each represented in a separate thing in the background (piece set on the rotating (anti)prism, speed level on the background colour shifting from blue to red), but it could be an interesting challenge for me and a nice touch for the player to have the music change based on speed and piece set.
I can't promise anything for now, but I'll keep it in the back of my mind. Thanks for the suggestion and the feedback.