What I would like to see in Windows 8:
1) Completely rewritten code. No more NT code. No more 9x code. No more DOS code. Build it with security in mind from the ground up a la UNIX.
For legacy apps (which, in this scenario, would be everything, a la the switch from MAC OS9 to OSX), build on the XP Mode from Windows 7 and call it "Windows Legacy." If a Legacy Mode gets infected by a virus, it can be temporarily disabled and cleaned off from the outside-in without having to deal with locked files.
Of course Windows Legacy would have to have better 3d emulation to run games...
2) Go back to what Windows 98 did during its installation process/what Windows Server 2008 does with roles: Typical or Custom install and the ability to add Roles and Features later. For minimalists, components they don't want don't get installed this way. For the average Joe, they can have a fair amount of extra stuff. Any program installed to Windows 8 will prompt for requisite Roles/Features/Services and they would get installed either from a compressed .wim file or from the installation DVD.
3) Take Windows 7's jumplists and put them everywhere. No more taskbar, Start Menu, or anything like that. Get most places in 1-2 clicks. One Jumplist could be called General with what would normally be seen in Start -> Programs; while one could be called Drives with the drive letters; another could be called Internet with IE, Firefox, Chrome, etc. listed; and one could be called Windows Legacy with "Legacy 1 (DOS)," "Legacy 2 (9x)," "Legacy 3 (XP)" as choices.
4) 64-bit only, as others have said.