Maybe after acquiring a specific skill, idk.
Like the mechanic ender's client is using, there could be a button showing each character's map, witch will be developing according to tile revealing when exploring around.
This way each character could keep a track of all the places he/she has been to and will also have a better feeling of exploration.
Now if someone goes to a tile you have already explored and pave it for example, this will not be automatically update your map until you pass by there again, thus you will "remember" tiles like they were when u were there.
I know this could require lots of data for each toon, but it could be stored locally in each players game folder, also giving the option to delete ones map.