This allows you to build a compact mechanical keyboard with a Feather controller attached to one of the sockets. It also has another socket, where you can put another board you like (both are interconnected).
The key matrix is interfaced through two I/O expanders, one for scanning columns and another for reading back rows. They use I²C with customizable addresses, so all GPIO pins are free.
The zeroth revision of the board is complete! It's enough to allow me to test it and find errors, although I don't expect a major redesign, just lots of polishing before the first release.
I'm waiting for some prototype boards and the needed parts to arrive, I'll upload some photos of the real thing when I assemble the first boards.