Search says no one has suggested this before, but it's kind of the most obvious thing ever: maps.
We have parchment. We have inks and pigments. We should be able to create maps like we create material accounts or hearthling descriptions and trade them to each other to expand the known world displayed by the client. Importantly, it should keep any notes or whatnot observed so that people can relate meaningful locational information asynchronously within the game.
Each map keeping a localmap worth of cartographic data seems reasonable as a limit.