A few designers have experimented with interaction between play by post players and their audience.
I think Epidiah Ravachol's motif writing ritual for PbP Swords Without Master is a great example, because it let's the readers of the story express their own interpretation of it.
I want to let more contribution though.
I'm running a play by post PbtA game on rolegate.com based on Paul Czege's The Clay That Woke RPG.
We are experimenting with a feature which lets readers and outsiders contribute to the story. On every weekend we hold an open discussion about a civic issue which came up during gameplay. You can share your opinion on it as a nameless minotaur if you have a rolegate account. Just search for Minotaur World and open it's public chat!
Does anyone have another example or idea?