Some of the later NES/Famicom games contained not just ROM, but coprocessors for better sound and graphics than the original console hardware could provide, such as the MMC5: https://wiki.nesdev.com/w/index.php/MMC5
Did the console itself, the cartridge slot, do anything special to support these coprocessors, was it farsighted planning in the original design? Or is the ability to add such coprocessors later, an automatic consequence of the ability to address ROM? Looking at the pinout https://forums.nesdev.com/viewtopic.php?f=9&t=14924 the most suggestive thing I can see is IRQ; it does not seem that should be necessary just for ROM cartridges?
(I get the impression that by the time the SNES was designed, they were explicitly thinking about later coprocessor support, but I would not have expected such thinking at the time of design of the original Famicom, which is why I am curious.)