The advantage of the tried and true aviation piston powerplants is that they are just that- tried and true. What annoys me, is that when you buy a brand new Cirrus or Baron or whatever you are effectively getting a 1930's engine with 1950's fuel injection in a horribly underpowered application. 8:1 compression? Come on. Again, the advantage of this is that the engine is so understressed it can't hurt itself, but the downside is a lack of power, and even if you don't care about power and climb performance and going faster, it hurts efficiency.
Anyone who is mechanically inclined understands the difference between a road car, which may use 15% of its maximum rated power at normal cruise speeds (and is expected to last at least 100k miles) and an aviation powerplant (or a locomotive engine) that cruises at 50% to 70% of maximum rated power and is still expected to last 2000 hours. Race cars produce significantly more power than a road car, but the motor only has to last a few races, and even then, failure rates are pretty high when measured as a percentage.
However, an interesting argument to make here, would be to use a higher compression ratio in the cylinder to enhance efficiency, yet limit the throttle body from ever being wide open. Doing this, one could arrive at say a 360 cubic inch engine, running say 11:1 compression, that produced say 240HP at 25 inches manifold pressure, and yet could still give the same 120HP for cruise flight, at 50% power, in a more efficient manner, than the current engine (O-360 at 180HP) running 65% power to make the same cruise power setting. This would give better takeoff and climb performance, and allow more efficient operation.
In terms of piston development, I think we'll see aviation diesels burning Jet-A before we ever seen a modern Otto cycle engine. But, for an Otto cycle aviation engine, liquid cooling the jugs would make for far more consistent operating temps and get rid of the problem of shock cooling. A higher compression ratio would engine maximum power and improve efficiency. Modern, port fuel injection (or perhaps even direct injection) and digital ignition with the ability to advance spark would help power and efficiency.
I don't think a Wankel for aviation use is a bad idea, mainly because it is just that- a rotary engine that doesn't have the enourmous loads of the piston reversing direction thousands of times a minute, but I think the real milestone, if we ever get there, will be the availability of a turbine engine for under $50k.
For what it's worth on the price side of things, I think that to keep GA alive, something has to be done about the cost of certified aircraft and the parts to keep them flying.
BEHOOOOOOLD.... 
. He also has an RV6 with a rotary and he's been flying it for about 15 years.