Eh, PPC doesn't run 680x0 natively, does it? Doesn't MorphOS and AmigaOS4 have an emulation layer for this? Why can't AROS?
AROS is "AmigaOS" on x86_64, and it is a really fun project, I have learnt so much from it and a really recommend trying it out!
Thomas is correct though, from an architectural pint of view the design goals of AmigaOS are far removed from what is needed in an operating system in 2017.
As for adding an inbuilt 68k emulator to a x86_64, I tried it, and hit two problems: endianess (which was quite easy, but very time consuming to solve) and address space size. Unfortunately as soon as the OS and the app operate in different size address spaces they can no longer share data structures, which is a a key architectural design feature of AmigaOS (other operating systems are far more strict about control of the address space).
You can read my issues here:
http://www.amiga.org/forums/showthread.php?t=72059