This doesn't seen like it would be too difficult to implement, but then again I don't really want to push peppy to devote all of his time on our requests. I do support you though, because this is simple but creative.
Really wanna do some awesome stuff with this. But either make it skin only (as animations on certain stuff are) or make the "bad" sound on the standard skin sound positive enough. Would ofcourse not want to disencourage someone who's goal was to finish a song just because it was a D ranking.
This is what I'm missing the most making the transition from Chunithm to osu!mania. It feels so satisfying hearing "Full Combo!" when you FC something.