I don't really know how hard this would be for you guys or if you already have it on your radar but I have a suggestion with rifts/breaks/etc. It would be nice to add a feature when creating a song that allows you to take one rift, and put it halfway into another rift, add rests which will cut a rift short or put a break at a certain point in a rift.

I have played a similar flash game to this which had these features but they were extremely focused on song creating and were around for 8-10 years (between two games).