Like, 4, like in Atari :) For 100% compatiblity :)

I knew what kind of magic could be done within 64K of Atari or Commodore. Include a 1M Flash and you can fit 16 RAMfulls of data :)
I would even consider it quite cool to have a DIP socket for programming eeproms as a "removable media" device :)
Especially if you could plug in an extra '51 and harness its cycles to do some extra work :)

As for different derivatives - I didn't consider availablity a problem - this isn't quite as bad. More I thought about incompatiblities in software and hardware. But they would save a lot of external parts. Think DAC/ADC "microconverters" for audio and video, hi-speed for CPU, multiport/USB/ethernet/multi-UART for IO. It's primarily the hell of programming tricks needed to perform ISP that would hold this idea back.