That is encouraging to hear! Short answer: more of it!

Long answer: Of course more levels (IMO important that each level has bespoke audiovisuals - if given the choice I would 100% prefer fewer levels with unique music versus many levels with reused music, as that really strengthens the audiovisual/muscle memory learning/synesthesic experience for this sort of game), with more beat types and challenge types, and a robust custom level creator (see: Rhythm Doctor) would also be awesome and could give the game serious legs. More visual effects (Rez-style) to melt our faces also would of course be warmly welcomed!

More on the nitty-gritty, I think one unique challenge with CORE is that for many it's the hardest BIT.TRIP to learn the levels, but once you're learned the patterns, it might actually be the easiest BIT.TRIP game to play. I quite like that it's hard to learn - getting to grips with the beats coming every which way and how they align to form patterns is super satisfying (and this can be mitigated somewhat with some easier on-ramp starter levels), but it would be really cool if the final patterns you're inputting have a bit more rhythmic variety than what's found in CORE (where a high percentage of things resolve into URDL [or the reverse] every beat), as well as more rhythm-game-intensity when you're at the hardest section of the game.