Something that has a tiny soundcard maybe, to work as an audio interface. Then you could use the GPD laptop for music more easily.
There is a device called the BOSS BR-80. It has two audio ins (stereo 1/8 and mono(guitar) 1/4 jack, and you can plug this in using USB to work as an audio interface, so when you use a DAW in your computer, the BOSS BR-80 handles the sound processing, which is important because if the computer has to do it you generally can play much less complicated sound (less tracks) before the audio crackles. Maybe you could make a module that just had the 2 jacks and the soundcard and it might be small enough to be a KVM, and it would appeal to musicians.