To make a project like this useful, you either need to spend a fortune adding all the sensors and 'radios' that someone might want/need, or give the mechanism to add them as needed.

Most systems use Qwiic or STEMMA connections to add additional sensors, but that really limits you to 2 pins, so it is relatively limited unless you use multiple plugs together (if you wanted something like SPI for example). This then opens up issues with getting plugs the wrong way round and things not working as planned.

I was hoping to push this towards the education sector, so using the Raspberry Pi style exposed header is open to damage by a bored student.

I therefore decided to switch it round, so that the more fragile header pins are on the cheap add on board, and the more robust socket is on the more expensive device. To make assembly simpler (cheaper) the header boards are mounted on the back of the board, using specially designed 'Bottom Entry' headers

The pattern of the sockets helps to make sure the add on board cant be inserted the wrong way, and limits any possible confusion. The case has a recessed corner with just a few holes as the only clue that there is something there

The add on boards only need simple headers to enable connection, giving access to power and ground, and 16 i/o pins

using thru hole headers means its more robust, but also could result in the weak point still being the more expensive device rather than the cheaper add on board