To me its extremely simple.
Everything possible needs to be ripped out of the OS, and made into apps. Mail, Music Client, Settings, Office, etc etc. Then there is no reliance on WP updates, no carrier issues, etc.
Nokia have already shown this is possible, with all the various little apps they have for audio settings, accessibilty, etc.
Make the OS a bare hulk, with APIs for everything, then update all the features reguarly through the app store. This seems to be the case for Windows 8, so why not WP8?