many people (including me) would like this feature, but I guess it is a problem to assign layers of different riffs to tracks. Here is my suggestion: use the icons in RW. Layers with the same icon will go to the same track. If there are two layers in a riff with the same icon then RW will create a new track. Song layers will also go to separate tracks.
Something like that.
I hope you find it interesting
cheers
juan
