Well, I wouldn't have enough interest in or knowledge about Rhythm Heaven to make such a site. And based on the amount of activity the section here has, I'm guessing most people on this site wouldn't join it either.
But if someone else wants to make one, go ahead. Could be interesting to see how big the series' fanbase is.
Even taking into account that the fans have already found their place, Rhythm Heaven doesn't really have enough to discuss for a forum dedicated to it to work. Wario has a world with well defined characters and setting (for Mario standards anyway), Rhythm Heaven is largely just a bunch of disconnected Rhythm games with no real plot or truly defined characters tying them together most of the time. Yes, there's definitely stuff to discuss, but I don't think it’s really enough to hold up an entire forum.