A would be nice However, for all I know there already is one but I don't know about it yet.
I'm not sure what a functionally complete set for mobile devices is, but having that universal back button on car dashboards, remote controls for media boxes, Android and iOS phones makes me feel like it's part of any functionally complete set.
I'm getting by without it, so that may prove me wrong, but I have run into many instances where I had a desire to jump one screen back in an app and couldn't. Instead, I just close the app and start over.
I see the value in having more screen available for all the device's useful functions by not devoting the space to the typical Android 3-bottom soft-buttons. This is why I said something about a gesture instead of a button.
- A tight triangular 3-fingertip swipe from right to left
- A single finger swipe starting at the base outside the screen and curving to the left side off of the screen
- A touch onto an inactive part of the screen that then travels up to the top left corner off of the screen
- An angry shaking of the phone oscillating from right to left
- A side of hand swipe across the screen from right to left
I have no idea how computationally expensive it is for the OS to be waiting for a gesture, so I realize these ideas might be wishful thinking or just crap.