I find myself basically looking at a few major sources for storytelling mechanics, here. 

One of them is T.W. Wombats' Historia Obscura, which I think has a really robust and interesting system for telling stories communally, particularly because questions of veracity/credibility are built into the game already. It's a really interesting read! What I'm struggling with is making it less contentious, as the players are most likely working together to convince the New Pope of the miracles, not necessarily competing with each other for the best story. Additionally, I'm thinking about what role the New Pope has in this -- is he the one challenging the players, "testing influence," for example? 

I'm also reading a lot about Matrix games, and this is a nice bundle of articles on them. 

Everyone is John has caught my attention, but not because I want this game to have a similar premise so much as I'm curious about exploring the idea of a character losing control of a narrative and taking over from someone else. What could trigger that? How would the players respond? 

I also like this essay from Legendsmith differentiating storytelling games from RPG's.

Besides that, I'm just looking at theater games. Good News/Bad News is one, where players in a circle take turns telling a story by alternating highs and lows in the story. Very structured, very straightforward.

What are some of your favorites?