Like there can be some sort of sign posts in each location, and people can post their messages. Of course there would be validations like no links, no invitations, etc. When connected to the internet, people can refresh the board to see comments from other players.
Or maybe a special crop that drops a seed. Seeds may be used to plant messages in places.
The issue may come in moderating when it comes to different languages, so maybe limit it to a select few languages...dunno. Was just trying to make a suggestion based on the "make the game multiplayer" demand.

