The next generation of windmills for harnessing wind energy is the BAT: bouyant airborne turbine. It has to be tethered to the ground, in order to transmit energy, but it reaches much higher altitudes where the wind is stronger. Its design prevents planes and birds from flying into it, but what about its cable tethering - which could snare flying objects?
Over the next few decades, we can envision our skies full of traffic as we become increasingly less earthbound. So I hope regulatory bodies and law enforcement are keeping up with the innovators!