Well, if it's going to be compatible with normal 68k Amiga software, it's got to at least contain the reimplemented Kickstart libraries. Like the old Macs with the Toolbox ROM, that's a significant portion of the OS (kernel and a number of driver, graphics, and windowing libraries.) I don't know enough about AROS or about the plans for this to say what portion of AROS that is, though.