Most App buttons today are designed horizontally. Stretching across the screen to allow for symmetry and a limited ability to use the thumb. But with the tap of just-2 buttons becoming the standard should we not work to optimize a one-handed UI?
So I have started to incorporate an idea I had into my projects. It is a Thumb-Biased GUI. What I have done is made the GUI adjust to the dominate hand of the user. If I have an app that simply needs the user to tap a few times for approval or the like, then I create, instead of a large horizontal button button, a tall vertical button set to the side of the screen near the thumb.
This system let's the user operate the app with the hand holding the device, with little to no fear of smearing across the wrong buttons, especially if there is only a single button. And the user doesn't have to work as hard to reposition the phone so that their finger aligns with the buttons.
You can see a general outline of my GUI concept below.