After posting this project on the web I've had a few questions about other use cases for this device:

Can it be used as a VR controller?

Not really because it only has a 6-DOF IMU, and for a VR controller you would want either a 9-DOF IMU (with a quaternion-based Kalman filter or similar) to sense the position or a different position detection sensor. If you added this to the board yourself then I think it actually would be a good VR input device, since it gives you multiple keys and/or analog inputs.

Can it be used as a basic chording keyboard?

Yes, it would be very easy to use it as a 4-key chording input. Just modify the lines at the bottom of main.cpp to processing multiple variables such as:

if ( (fing1 > FINGER1_PRESS && !press_keyup[1])
  && (fing2 > FINGER2_PRESS && !press_keyup[2]) ) {

I personally haven't used chording keyboards so I haven't implemented it, but there's no reason for it not to work.

Can it be used to control a quadcopter drone?

Yes potentially, either by using the analog finger positions or the analog motion inputs. But since the code is configured to send BLE keyboard or mouse inputs, you would need to do some work for it function as a drone controller, unless if you find a drone that already takes BLE keyboard or mouse or joystick inputs (I'm not sure if that exists?). Using it as HID Joystick to control a drone flight simulator on a PC would be a relatively easy option.