obstacle avoidance for rc airplanes would be nice, maybe combine it with one of the autopilots out there. Learning to fly rc airplanes is pretty easy these days, the flight controllers can hold your hand through just about everything.

I built a FT Explorer with their FT Aura gyro system and it's so much more fun flying in a gentle breeze when not having to worry about keeping the plane upright and level. Radar ground detection and obstacle avoidance would be a crazy cool addition.