@AppLee My suggestion was from the bottom, not from the top. I agree that it would have to stay within the current framework, and from the top would make no sense for usability either.
As for it being more complex than a button - I see the point from a developing perspective, but I disagree from a user perspective. The back button is usually found on the top left, which is the least accessible point of the screen.
Assuming this is a feature that would be useful but is difficult to implement, would it not be possible to write a standardized function of sorts that app developers could choose to implement in their apps at their own free will?
If it would be good for users but complex for developers, maybe it could be possible to aid the developers by giving them some sort of tool box. But of course, I don't claim to have any expertise here - i simply think Username's concerns are valid, and made a suggestion for a possible solution.