It would be handy to be able to place notes on hexes. The note could pop up on hover. Notes could be restricted to a character count to stop the pop-up causing too much trouble. A hex with a note attached could have the same border colour except slightly thicker. However a thicker border would not work well for hexes containing a planet.
I grant that a big question would be the control you use to create a note, what with right click being for movement.
As an alternative, just a built in notes function that doesn't interact with the map would be useful. Some kind of simple text storage thing with an icon in the top bar.