Not terribly exciting, but a prototype to demonstrate that:
* interfacing the RPi to a MCP23017 provides 16 bits of buffered I/O ports
* multiple chips can sit on the I2C bus, yielding 256 IO ports
* 2N7000 Mosfets provide bidirectional level shifting so that the RPi (at 3V3) can talk safely with the other side at 5V.
* external power sourced (for higher current requirements) or 3.3/5V selectable from the RPi
Stripboard layout and some simple demo python scripts at https://github.com/rm-hull/mcp23017