Some features that would be cool:

• Battery-powered: the goal would be to have a really portable instrument, with which you can jam outside!
  I already tried with a single 3.7V 18650 battery:

  

  It worked well for more than 1 hour.
  But in order to have a really cool user experience, we need a circuit that would automatically charge the battery when a phone charger is plugged in (case #2 below), and that automatically switches to battery-powering when the charger is removed (case #1). The switching should be done without any 1-second-power-interruption (that would reboot the Raspberry Pi)!

  

  And a last thing: we should be able to detect "Low battery" and print it on the main display.

  Help wanted on these topics!

• Loudspeaker: find a small-sized loudspeaker + an amplifier that could be embedded in the box. LM386-amp or PAM8403-amp? I already tried a bit, but not really happy with the result.
  Should the headphones output also be amplified? If not, we need to turn the amp off when headphones are plugged, and turn it on when using loudspeakers (i.e. when no headphones are plugged).
  Again, help wanted for this part!