It probably depends upon what retro device in question but I'd look for - some LEDs (for the front panel and for debugging) - SD card (beats providing a real floppy drive interface or tape). Having the card change exposed would also be very useful because even the 8bit world supported disk change interrupts - At least one RS232 level shifter For emulating older 'serious' machines I'd look for - at least a second RS232 level shifter set (possibly at the cost of the joystick) - Real time clock (or perhaps just sneak a digital grove connector on it and avoid the costs/hassle for all), as they do an rtc - S100 connector ;-) When I think gaming I tend to think - one or two joystick - reset button - leds - video/keyboard/audio out, maybe audio in (for tape) but thats probably not a serious use case - keyboard (but not really mouse... mouse came very late and devices with mouse tended to be a serial mouse anyway) - RS232 level shifter (but much less important here) And my personal wish list would also include some kind of networking as a lot of these older machines did have some weird and wonderful networking options often with insane hardware which nobody in their right mind would want to emulate. Things like CP/NET for CP/M could use serial but there was also stuff like the spectrum ZX-Net, Research Machines Link 480Z etc. I think that depends what you are anticipating the usage model to be. If it includes being a good basis for things like ZPUino hacking and the like then networking is a good thing to have, if it's just classic emulations then less so. I don't know what the practicalities of making one or maybe both of the joystick ports jumperable as joystick or RS232 would be. Perhaps that would avoid all the problems around snap off bits. I think that would work well because you'd effectively have a choice of two joysticks, one serial (classic 8bit home micro etc) one joystick, one serial mouse, one serial (8bit higher end system - eg emulating the PCW512 etc) two serial, one joystick abused as four front panel switches (8bit era 'traditional' system) Btw - I don't know if you are familiar with the PIC32 based retro-home computer platform ? https://www.olimex.com/Products/Duino/Duinomite/DUINOMITE-MEGA/ but that's a fairly good example of the sort of ports that are useful.