I plan to have quite a few devices in my keyboard: a screen, a knob, RGB lighting… I haven’t done the final count, but I’m sure I’ll need more GPIOs than the ones available on the Pico.
The GPIO extender I’ve settled with is the classic Microchip MCP23017. It’s a 16 bits, 1.7 Mhz that works over I2C and that will add 16 I/Os interfaces.