My first PCB design is the backplane, as it is relatively simple and will be needed by every subsequent PCB. It took a couple months of research, but I settled on JAMMA style edge connectors giving me a total of fifty-six lanes between the cards. I decided to break six lanes out to separate "card select" headers, so only fifty lanes are tied in parallel across the edge connectors.
To keep costs down, each board contains only five JAMMA connectors, plus pinheaders. I should have designed interconnects, to make linking boards easier, but the end result is a backplane allowing a maximum of twenty-five connected devices. I will need to order additional (or replacement) PCBs to expand beyond twenty-five connections, but I do not expect to need more than twelve or fifteen.