I often have white lines as symbols on a map since they show up really nicely on an aerial background. Like most folks, my map layout is on a white background. The problem is showing these symbols on a legend, white is not visible on white! While I have rigged the legend by creating a copy of my layer and then adding a gray background to the symbol and making the item visible on the legend if the item isn't turned on, it is problematic when you want to only show the symbol on the legend when the item is on them map and not when it isn't.
I think this could be solved by having an option to add a shadow or a halo to a symbol in the legend as you can with text items. Below is a sample of my problem and a sample solution.