Just wrote one for fun. It's a vibrator massager app (which is not unique), but one of the differentiation feature is unique - Basically allows people in proximity be able to vibrate together, control each other's devices.
No annoying ads, all vibration patterns unlocked. Only premium feature is ability to host party. Joining a party is free.
What's next? Btw, I have some promo codes left, let me know if you have people want to try ;)