There's a lot that can be done if you don't need voltages above 5v or more than a few amps though, as long as you don't need the full flexibility of the MCU. I

There's a lot that can be done if you don't need voltages above 5v or more than a few amps though, as long as you don't need the full flexibility of the MCU. If you make all the IO pins accessible, you'll never protect them all fully or have any room for dedicated special purpose connectors, but if you choose the most common features and only support those you can do a lot. For the board I'm doing I only have 3x servo plugs or STEMMA connectors, plus an LED strip output, switched 5v output, and Qwiic/Stemma QT plug, plus headers for the most common class D amp and RS485 modules. That covers about half of all Arduino stuff I've ever done, and leaves a ton of room for onboard features even on a 35x70mm board.