Proposal for the module board:
Modules are arranged in a 2x2x5 matrix = 20 modules in a single PocketQub
Eight long pinheads go up through the five modules of each quadrant.
Here is a picture of the proposed pinheader,
Five pins are unique UARTs (halfduplex) for each of the five modules in one column, additionally a +3V3 and a GND pin.
I planned a unique power latching for each module, but this would require additional five pins. Instead, there might be a latching for each column, and to prevent other modules of this column to be shorted by another module, I´d suggest to use a polyfuse on each module mandatory.
Let me know if you got a better idea to arrange/connect the modules which saves more space.
NOTE: I don´t want a bus system, because I want to avoid unnessecery coordination between the modules´ owners and avoid a bus failure if one module shorts the bus.