We have some sorts of similar things. EyeToy and Kinect were additional hardware added to the system for games; not on the same media as the games though. The difference is expansion hardware today is always going to be subordinate to the main system --- the economics and data transfer realities don't work out to put a polygon processing enhancement in a USB add-on; but the cartridge slot has immense potential and it could also be cheaper to use the base system with enhancements than to release a whole new enhanced system (although, it's not clear to me how much the base genesis/megadrive benefits the 32x)
Well with xCloud coming up from Microsoft, and Sony partnering with them on cloud gaming - we appear to be going in the opposite direction, for better or worse.