Right now I'd be happy with something like the C64-DTV in an Amiga version:
A cheap A500 clone on a chip that we can hack to add a PC keyboard and joystick. Perhaps with a place on the motherboard to add some sort of busboard for at least 1 Zorro II card. Oh, and a built in flash memory chip or slot for memory card to act as a hard drive.
That would be a good start. Not too much to ask. Not over-ambitious.
Once we have that established, then better versions with faster processors, more expandability, RTG graphics and sound, etc.