I'm too lazy to look up the details now, but I'm pretty sure the SNES almost sort of did nearly have an add-on. Sony and Nintendo were looking to create a CD-ROM add-on / upgrade for the SNES. I somewhat remember the rumors from back in those days and I was, at the time, very excited about it. Granted, my parents wouldn't have been able to afford it, so who knows if I would have gotten to play it. I'm still bummed that I never got to play the Sega CD or the 3DO. Practicality and foresight aside...
I think a CD-based SNES could have been pretty cool, especially if there were some additional technical upgrades (i.e. updated processor / 32-bit / etc). I suspect that the library wouldn't have leaned so heavily into full-motion video like the Sega CD, and certainly the first party Nintendo titles wouldn't have gone that route. SNES had some great RPGs and fighting games and tons of amazing first party titles, so long as the system had enough RAM, those genres would have benefited from more storage. Had there been extra processing power, that would have been a nice step up as well.
On the flip side, Nintendo hardware is basically just a door to the Nintendo software experience. Lots of great stuff, don't get me wrong, but also lacking. So, the fact that the Sony collaboration fell through and ultimately resulted in the Playstation probably worked out so much better for the video game industry and gamers.