long time ago, I played a rather obscure game called Gurumin.
It's an adventure game that mixes RPG elements with platforming elements, it's vaguely similar to a Zelda game (gameplay wise).
It's interesting for the way how the game manages critical hits:
Actually, this is true in every version of the game, but only the PC version has an indicator for it.
The critical hits are rhythm based.
You have to attack with the correct timing in order to do a critical hit, and the timing changes depending on the music that plays in the background.
Honestly, the concept is very good on paper, but in the game it was pretty confusing and not very well implemented... mostly because of the lack of feedback (sometimes a move is laggy and it's unclear if the game considers the moment when you push the attack button or the moment when the move hits, and so).
The feel while playing the game was that the critical hits were coming out at random.
But I'm sure that with some focus on making it more intuitive and easier, the concept can be improved and better implemented in a game.
Why I'm saying it here... well, pretty obvious.
What do you think about applying this to a Rhythm Heaven character in Smash?
As a big fan of Karate Joe, I think that Karate Joe would be the perfect character to use this type of gimmick: a character with a simple moveset so you can focus on the rhythm gimmick while playing (but it can apply to anyone, really).
It would be very hard to balance it, because the character would become stronger or weaker depending on the pace of the song playing in the background; there is also the Pikmin stage and the environmental sounds... well, during that "song" just put a metronome sound.
As unbalanced as it may be, It would be hilarious to see tier lists where the rhythm heaven character using this gimmick is placed in a variable position depending on the bpm of the background music