If you do think of adding historical sites (battles, dungeons, etc) you could do them in white (or map base color) and a black outline so that your population centers stand out more, but you have more details for a world history flavor to it. Those don't necessarily have to be added to the Legend either. Mark a battle with crossed swords, mark ruins with something that looks like one (greco-roman are easy to identify as such, but there are others), a tree if you have elves and want to designate a major area for them...