I was thinking of a group or cluster of quadcopter flying together.

There are some nice protocols and topologies in Wireless Sensor Network topic. While devices' sensors collecting environment data they can communicate with each other in several ways (e.g. ad-hoc, hierarchy) and command each one to behave in different ways (e.g. quadcopters maneuver in whatever beneficial pattern)

Some more ideas on that flying object example, it could calculate overall battery usage and balance it all over cluster by wireless charging on the fly.

Underwater devices or robots would be more interesting.

Well, one obvious example is the space probe(rosetta) that use sensor to detect comet's surface and sent data back to earth: http://kdvr.com/2016/09/30/rosetta-landing-comet-chasing-pro...