1) Fix Reason's non-standardized and often unhelpful parameter value displays. I've already complained about this but it's worth repeating. Really, it's one of my biggest Reason pet peeves.
2) Multiple audio lanes per audio track (sequencer feature). Imagine two or more clips side by side in the sequencer, each with their own volume and pan controls (simple knobs would do). This wouldn't be a group substitute, it would be a convenience for double tracking and stereo pairs. It wouldn't really add capability but it would be a major workflow improvement, streamlining several processes.
4) Midi Out.