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.