.A shortcut and menu to reset image to its original size (ie: depending on the mode, it will be image's original size, or win size if scale is enabled)... I suggest '*' key that is already a standard in all platforms/viewers.
.Easier way to switch between FS & Winmode: double-click on picture for example, as well as 'f' or 'ctrl+f' or 'alt+enter' key.
."Real" FS mode :) (no horizontal bar left on top of screen)