This is the new design. After a few years of inaction I've come up with a modified design that is much simpler and yet more powerful in most ways.

The original design only had a single solder-on side, this new design has four, all of which can house electronics and/or connectors for IO interfaces.

Another big change is the processor. Despite the fact that I love their chips and tool chain, after total lack of support from NXP I've gone with an off-the-shelf design in the form of a tiny carrier board with a ATSAMD51 Cortex M4. This is an Adafruit product that has good support and is Arduino ready. Therefore I will spend almost no time debugging the processor electronics and fighting with the development toolchain only to wind up with something that nobody will want to use because it's not "Arduino inside".

Of course, as the processor is on a carrier board, and the backplane is very generic, a new processor version can be designed at any time and just plugged in.