I would love to have the ability to have multiple versions of the same passage in one file, operating a little similar to cues.
Versioned fragment is highlighted, there is a dropdown with names of the versions to choose from.
For an added oomph - the region could differ from player to player - one person has a half-bar pickup, others: a tail. It shows as one group, and has the ability to do A/B testing, preferably during playback.
This is something hard to implement if I am not mistaken, but would enhance my composing workflow immensly by removing the logistic problem of trying out modules of ideas in a context. Right now if I want to have a second pass I need to duplicate something - flow, file, player, bars. Or I have to keep extra bars at the end of the flow and then manage them and whatever is forgotten on them 
Getting this out of the way would mean I could sketch faster and with less stress - which would mean that some things would get the second pass they deserve - where in normal circumstances they would not get it.
Nikola