I never got it too, to be honest, though I have a feeling it blew up due to the Gematsu leak, as
Ramen Tengoku
said. However, someone mentioned a possible gimmick, that while I wasn't a fan of, I thought about it some more and how to improve it. There are some rhythm games, not RH mind you, where you can put in your own music and it has software that converts it to a pattern that can be played as a game (e.g., Rock Band clones). I wonder if it's possible to do a similar type of thing for a RH moveset, where the stage music creates a pattern, invisible to the player, but that if you sync up your attacks to the beat, causes extra damage or something special to happen. For some stages with purely ambient music, the character would still have their attacks, but wouldn't get those buffs, and in particularly chaotic music, they may be overpowered as a result. It would be chaotic, like Hero, some people would hate it as different stages with different music would cause different results--though, in a predictable fashion since the pattern is the same for each song. But it does sound fun from a party game perspective, and definitely not from a competitive one.
I'm not sure how much that lines up with Rhythm Heaven, as that is a very different type of game than most rhythm based games. But thinking about that made me think that would be a fantastic combination, actually using the stage music itself, which is just aesthetics and has no practical purpose right now, to have an impact on the gameplay. Probably wouldn't work, probably doesn't faithfully represent Rhythm Heaven, and there would be people who wouldn't like that gimmick, but that idea made me go from "why?" to "that actually sounds awesome." I'm sure even if this doesn't work, if Sakurai did decide to have a Rhythm Heaven character, they would come up with a better idea that actually works, but that opened me to the possibilities that type of character could bring.