COST - COST - COST
1. Reduce weight and manufacturing costs. Twenty years ago I read about teethless gears that would drive through pressure/temperature "morphing" fluids that would act as adhesives when in gear contact and as lubricants when not. I'm still waiting for this. But it could reduce gearbox weight and cut gear production costs substantially.
2. Reduce engine specific fuel consumption. Again, twenty years ago we were talking about 50 lb. engines that would produce 500 hp with internal ceramic components running temps close to 2000. This should be the primary focus of the entire industry. It could make possible a twin that weighs less than a single.
3. IFR improvements...increased stability and control and known icing capability.