Roll20 could use another layer of fog of war for already explored areas just like in computer strategy games (Age of Empires etc.) A tool that adds and removes this fog of war is really useful for situations when NPC units leaves the immediate encounter area. A Gm might still want to keep track of a unit when it disappears in line of sight (he might want to bring the unit back to sight from a completely another direction later!). The problem with the regular fog of war tool for this is that it hides also the explored area itself. I, personally, would still like to keep the players aware of their explored surroundings but in case of line of sight issues, I would like to be able to hide the movement/changes in these areas.