Use newer and smaller barometer (BMP180? LPS25H?) -- DONE! (Using LPS25H[1] now)
Smaller bluetooth module with BLE support (CC2541 with BlueBasic or nRF51822 with blessed or nRF8001) -- DONE! (Using nRF8001[2] now)
Use SMD vibramotor. (But make sure that it does not get stuck)
Reduce size to fit into 5x5cm (4x7cm currently) to reduce costs. 4x5cm would be great! -- DONE! (4x5cm)
Use combined accelgyro and magnetometer (MPU9250[3])
Use LPS4012-222L or LPS4018-222L instead of huge inductors. -- DONE! (L-07C15NKV6T and LQM18FN100M00D)
Smaller transistors -- DONE! (SOT-723-3)
0402
Charge LED is disabled when the device is turned off. This leads to false assumption that charging is not working when device is turned off by hardware switch.
Increase reset button threshold (it is too hard to reset the device without going into flashing mode).
WARNING! Current leakage through STM pin protection diodes when powered off. This causes standby I2C voltage drop and incorrect current measurement by charger chip. Issues need to be solved as current ProgArm 3 is unsafe to operate! Temporary solution: to cut the trace that connects R-ilim to STM32 pie.com