Support for a TEMPEST mode of communication would be a killer feature. Perhaps vibrate mode on one phone being picked up by the accelerometer of another? In our hypothetical dystopian future The Regime will probably jam 2Ghz to 5Ghz in public spaces. TEMPEST mode would also force them to install vibrators into all coffee shop tables.
You are misusing the term. TEMPEST is an attack.
> Perhaps vibrate mode on one phone being picked up by the accelerometer of another?
At very close distance vibrating our vocal cords and eardrums would be much easier and works without battery.