Remote Talk Cap
Making a narrow angle modulated light emitter and receiver headband, connectable via bluetooth to the phone, would make it possible to communicate with people at line of sight, directly choosing with one's head (like a laser pointer), whom to talk to.
Dolphins are known to carry something like ultrasound lasers, so they can emit sound directly connect to a specific another dolphin, and these signals may be modulated, creating the ability to engage in targetted communications.
The remote talk cap would work in a similar way: you'd have a head-band with a kind of directed light projector in front, which you could focus to an arbitrary other person, and if that person wears a similar head-band or cap with a receiver, he or she would start hearing what you say over a sizable distance. You could also adjust the width of the projector, to include larger areas (with lower power density).
What could this be useful for? For communications at line of sight. While, perhaps modern people are not so keen on communicating with local others, but it may be something educational, recreational, and sometimes pragmatic, for example, for neighbors under a lock-down due to pandemic. ;)