When handsets become 5-6 inches, and our hands no longer cradle the phone in our palms, the bottom navigation keys make less sense. Windows 8, designed for larger screens, of course already understand this, and uses edge gestures extensively. We of course already use an edge gesture to swipe down the notification centre, and use it to indicate Back in IE11 also.
I believe the task switching gesture already used in Windows 8 would work well in Windows Phone 8, and with the upcoming merger of windows RT and Windows Phone in Windows 9 it may come automatically to Windows Phone.
