When in presentation mode, the zoom commands are disabled. I'd like to be able to zoom in and out of the display while presenting. That way if I'm presenting something much smaller than the display resolution, I can zoom in so people across the room can see it. Alternately, it could be used to zoom in on an area of a larger design to focus on some detail.
You'd need to address panning (space bar/hand tool style?), but even just relying on scrolling via the trackpad could work to start with.