Years ago, before all this electronics boom, I thought on making a board full of pci connectors, all pins connected together in a bus, then making a master card (CPU, Pic, avr, whatever) and a bunch of prototyping PCBs where you would solder all kind of stuff, having all signals available there to use, avoiding lengthy messy wires everywhere and allowing quick removal or addition you would insert any card in any slot indifferently.
The idea was to avoid the usual problems in breadboards and to easily add and removed modules.
Well, just like today with Arduino hats, but better.
However back then PCBs were no close as cheap as now and didn't pass the idea stage.