FPGA ftw! 
An x86 on a card running an emulated 68k that appears to the Amiga as a real 68k CPU is a fantasy I've often dreamed of too. Theoretically it would be faster than any current FPGA tech.
Ah we can but dream eh?
I think someone at A1K.org did this quite some time ago with an A1000. They replaced the 68K with a whole bunch of wires and other logic hooked up to a PC. Cool proof of concept. Something more practical and self-contained could probably be done nowadays (probably with ARM, as another post mentions).
