Hideable, Moveable, or Floating Toolbar
Being left-handed, I find writing at the top of a page to be very frustrating since my hand rests right over the back button on the toolbar. I'll often get kicked out of a doc from the side of my hand hitting the back button once my pen is far enough from the screen to stop rejecting touches. Being able to move the toolbar to the left or right hand side of the screen and/or being able to hide it altogether would be a great feature to resolve issues like this. I think above both of these, a floating, minimizable toolbar would be extremely useful, but as long as I can move the toolbar away from my writing hand I'd be satisfied.