The ability to stack plugins to put a MIDI effect before an instrument. Consider Bitwig, Reaper, Ableton, probably most every other DAW that allows this without having to route it. This could be done by simply allowing instrument vsts in the audio effect area. In the same vein, if this is not a possibility, the ability to send and receive on multiple MIDI channels to be able to facilitate MPE, specifically for handling per note pitch bends which is not standard in MIDI hence the MPE workaround.
I love the overall approach of renoise. The amount of control over individual parameters is great. I love the tracker style. Just wished the above was possible.