I like that idea too. I mean, a '51 IS a "bad choice" for a microcomputer as it's a microcontroller, not microprocessor. But it's still pretty much possible, and with the extra pins it could go with real 64K RAM -and- some ROM, IO etc, not overlaid on the userspace memory. The Harvard architecture would be a pain in the neck unless you use the "XRAM=CODE" PSEN+RD trick. I even think it would be nice if it came with a semi-OS (NOT multitasking!) in ROM that would allow you to load the program into RAM and then switch the ROM off so all the ROM would vanish and be accessible only through special calls/invocations.
The tricky parts I already see: Mass Storage, Video (would be damn neat if it could cooperate with a TV set like the Ataris did), keyboard. Everything already known, already possible and already quite bulky and heavyweight :P