The Meaning of Life, the Universe, and Everything.
Basically, I think it would be a fun mechanic to be able to either put a lead or a fishing line on another player with elytra, and when you run they are pulled up into the sky and towards you (you know, like a kite). The elytra player could either maintain control, but be unable to leave a certain radius around the player, or it could work more like the second seat in a boat, little to no movement control but you're free to look around and shoot things. This isn't intended to be a huge game breaking mechanic, just something funny to mess around with. The ability to stick a lead on another player could also lead (haha see what I did there) into other mechanics (keeping each other anchored in end cities, mountain climbing while tethered to each other, etc) but I'm more interested in flying my friends like kites.