OS5 will bear a resemblance to the earlier Amiga OS's and will bear ultimate scalability. It certainly will be more Amiga-like than MOS/AROS.
I'm hardly a MOS/AROS zealot, but I'm going to have to disagree a bit on this. OS5 would, I'm guessing, be based entirely on the new A2G technologies, and could only run OS4.x apps in a sandbox environment.
I like A inc's timetable through the 4.x series, but I have never got exactly how much of OS5 would be AmigaOS, or DE. Whatever happens, It's a bit early to be talking about OS5 now OS4 isn't even out :-)
