When I was a child in the 90s I somehow scored a voice role in a hotdog commercial for the radio. I was paid a king's ransom for this, half of which my parents made me put in savings (wise), and half of which I spent on a brand new Sega CD (not wise).
The magic of postage stamp-sized full motion video took about three days to wear off, at which point all that was left was basically pure shit. They jacked me. At least I learned that lesson early.
You know what's funny? Nintendo put expansion slots on the bottom of all of their consoles prior to the Wii. In Japan, they were used for the Famicom Disk System, the Satellaview, the N64DD and the Gameboy Player. The latter was the only one that made it to the West. They never released an expansion for a console outside of Asia. They even had to retool several games that were released on Famicom diskettes for cartridges in the West, including inventing on-cartridge save files via battery-backed RAM for The Legend of Zelda in order to release them in the West.
Given Sega's track record with console expansions, Nintendo might have been just as well off. Well, except for how the SNES optical drive add-on played out.
More directly. The playstation was developed as an expansion for supernes. Then nintendo changed their mind and sony decided to keep developing it as their own device