I have an idea regarding both of the mouse buttons that I thought might be cool. I always find myself switching back and forth between elements, which I find to be annoying when I have to do it repeatedly. It is also annoying to have to switch back to the eraser when I do decide to have two elements picked.

So therefore I thought I might suggest having a key or shortcut for each mouse button to swap between the last two elements selected with that mouse button. So for instance, you select DUST and SAND with the left click, and instead of having to reselect individually over and over, you use a key or shortcut to switch between the two. Same with right click. I think this would eliminate some of the annoyance experienced when you're working on something and switching between two elements and also (being human) needing the eraser from time to time.