An idea for a type of touch-screen gesture for phones: the user places their left-thumb anywhere on the screen and keeps it in the same place, and at the same time they swipe across with their right-thumb.
You can think of the left-thumb as acting like a modifier, like the shift-key. A left-handed person could keep their right thumb stationary and swipe their left thumb - it'd have the same effect.
Each different swipe direction (up, down, left or right) could be recognised as a distinct gesture with its own distinct meaning.
I don't know if anyone has used this type of gesture before, but the benefit of it is that it's quite easy to perform. Which is one of the challenges with gestures: finding ones that are really quick and easy to do.