Tough to say 1 feature, probably audio routing but I really want to see Renoise up to par technically with other DAWs out there.
Routing of MIDI to/from VST effects.
Routing of audio to VST FX with extra inputs (compressors, vocoders, etc).
http://forum.renoise…ing-to-plugins/
http://forum.renoise…502#entry312502
A resize feature in advanced edit
http://forum.renoise…ed-edit-resize/