Thanks to all for answering and letting share their ideas!
Yes - this will be an option. But having to support a software on different smart-phones is absolutely out of my scope Also I am not sure if this can be really used live on stage.
This should not be a show-stopper. In fact you won't really use a stack of more than two or three hats. This has side-effects to the cases - perhaps a standard case does not fit anymore. Anyway for my PiSound card the standard case will not fit due to the MIDI connectors on the board.
I think simplify wiring is the way to spread Zynthian over the world. For more controllers there are several additional options (MIDI, uC/USB, ...). The goal should be to have a well defined system which is easy to build and use. For me: Fernandos standard design with four encoders, soundcard and a display.
What I have identified in this discussion:
- Stage/live compatibility?
- Is compatibility to a case important?
- Is external hardware needed or should a smartphone/tablet (optional) be used as controller?
- Is an expansible controller system necessary or only the four /standard) encoders?
- Is the goal to simplify wiring or have more elaborate on box controls?
The MCP23017-hat seems to be a very nice toy! But you still have to create a set of cables for the encoders and you have to solder the MIDI input opto (also if using a uC for encoder-controlling).
Will a dedicated "all-in-one" PCB the best solution? I have something like a hat in my mind. One PCB with direct soldered encoders and MCP23017 plus MIDI opto. Only one ribbon cable to the Raspi and a cable for the MIDI connector... But one "big" PCB is a limitation for some cases or displays...and how to produce this PCB (Kickstarter?) very much problems... difficult to solve...