Adrian Manteuffel has created some concept designs that cover multitasking on the platform as well as further customization of Metro UI, which includes altering tile transparency, changing themes (beyond what's currently available) and applying wallpapers to the home screen.
The above concept shows multitasking added in on the right-hand side of the home screen, which is probably not a bad idea (if you don't mind losing more 'negative space' which the UI is famed for), but I don't see it working with a large number of apps open at once - I don't think a scrollable list on the side of the tiles is a good idea.
For the email tile that now takes up two positions, I don't like at all. I'm a heavy email user (compared to SMS anyway) and I prefer to have a minimal amount of tiles present. Unless that email tile can flip through the email accounts every few seconds then I don't see that being adopted by many.
See the other two designs after the break.