Warning: this is very long BUT I split individual ideas into paragraphs for your convenience.
Can you add a little line with a dot at the end that shows you where you will be if you dash? This would help a lot as the first boss wouldn't kill me as often in round 1. I'm good at this game... but only once I get past boss 1. That's the hardest part, believe it or not.
Also, how about there's a dash column where if you put notes in there it will upgrade the dash. (green=farther dash, blue = mini comet, yellow = small explosion after each dash, and purple makes you invulnerable after a dash for a second or 2 OR lowers dash cooldown.) This custom dash would make a 'Thump' Noise.
Can you add characters with different BPM, ATK, HP, amounts of columns, different colors (like instead of green yellow blue purple, they would have that double shot, fire yellow and blue.), starting notes? I would understand if you didn't put it in the browser version, but the full version NEADS THIS.
Can the vannila green PLEASE BE SPEED? We want speed back! I always use it!
Okay, so note color: Red. It would shoot like a sword out on your right front, than rotate like the 1st boss's laser, only it would be a sword? Purple would do swords all around you, blue would deal more damage and make it wider, green would make it longer, and it yellow would make a big explosion in front of you.
Challenge mode: Notes are dual-colored instead of white and can only be put in one of those 2 rows. You can also use these in the real game as a bonus drop that are upgraded notes, maybe even single colored.