The following screenshot (enlarged 2 times) illustrates two flaws in the existing world axis icon.
The first, most obvious flaw is that all axes are displayed in front of the letters (x, y, and z), regardless of their relative position. For example, in one of the icons shown in the following screenshot, letter z is in front of the axis y, but it is displayed behind the axis.
The second, less obvious flaw is that linear shape of the axes sometimes make it hard to see whether we are looking at the front end of the axis or its rear end. Static view is ambiguous, so we have to rotate the view to see orientation of the world axis icon.
The best solution of both flaws is shown in the following drawing of improved world axis icon. The icon has black center and tapered axes to make it obvious whether we are looking at the front end of the axis or its rear end. In the right image all axes are visible. In the left image the y (green) axis is invisible, but its orientation is obvious. The colors of the axes are easy to distinguish because the axes are much wider than one pixel. If the axes are much wider than one pixel, the x y z letters are redundant.
In my opinion, the world axis icon should be clickable. Left mouse click should toggle Fullscreen. Right mouse click should launch -Fullscreen command (Fullscreen with options).