Does know anything about users being able to modify/influence/change levels?

Ok so I have been developing some ideas to get to work on and I have one specifically that I need some assistance with. The App will be called “A Walk On the Beach.” It’s somewhat of a 3D representation of the Apple app “Calm.” The idea is that you can take a virtual stroll up and down a pier on the beach. Building the level of a pier seems self explanatory to me... but my question is this.... How could I make it so that players can leave notes on the pier for other users to read and or respond to? I was thinking something like a virtual “peg board” at the end of the pier where players can “pin up” pictures or post it’s. Basically my question is how can I make it so that one player can make a change to the level (leave a note on the peg board) and it will permanently change the level so that any other player that visits the pier can then see that note.

Any advice on how I could accomplish this would be helpful!