I got an idea that you can use an angled 5-pole female HDR connector on the controller and a male connector on the expansion modules as and in the accordance followed a female connector on the other side of the expansion module so you can just link them together

and in the HDR connector will be SDA, SCL, VCC, GND and Vin

I hope you can use my idea to develop the Industruino

It could be a good in it with a model without display and with knaper and RGB LEDs, instead of and to minimize the number of I / O pins you can use a PCF8574 is an I ² C bus to LEDs

I can self use one that has display, 3 buttons, Ethernet and RS485 (terminal) and have the opportunity to self add a TO-220 regulator, for I have 24V DC to another controller to run together