I like ideas, so dont get me wrong. Even if they seem ambitious. Technical issues aside, I dont think there would be a big demand for a multi-platform console. retro enthusiasts will get the individual consoles for nostalgia value. and you wont attract many 'new' customers, tbh. In fact, I doubt your device would sell more than 10 units, and Im afraid you need a lot more than that to make it commercially viable.
Last, just because devices share the same cpu or have similarities in basic, this doesnt mean they are easily made compatible. After all, games did not get programmed in basic and a lot of consoles have custom hardware. If you really want to proceed, then look at the Chameleon64 device: no cartridge slot there, but able to load disk/cartridge images, and the ability to act as an Amiga, C64 and whatnot, all from the same device.
