One of the elements I used in the Eighth Sea was a system where players could introduce aspects into a story. I did this at a fairly simplistic level with each player being given a limited number of tokens which could be expended to introduce anything into the events at hand (within reason). The context of the Eighth Sea is a time travelling swashbuckling epic.
If a player wanted to introduce something into the game that was suitable for the setting it would only costs a single token, eg. A paratrooper is introduced into a scene occuring during a scene in western France during WW2.
If a player wanted to introduce something that was a locational or temporal displacement, then they'd have to spend an extra token, eg. A kamikaze pilot or a Gaulish warrior in Western France during WW2.
If a temporal and locational displacement occured then a third token would require expenditure, eg. A samurai in Western France during WW2.
Such introductions to the storyline were simply surreal diversions which could then be used by the GM to weave new plots into the game.
If the player introducing the element spent an additional token beyond what was necessary to make the object appear, then the element stops being window dressing for the scene and it becomes an immediate threat to the story.
It worked for the Eighth Sea because the game was a very tongue-in-cheek setting. Anything was possible, and it was through the combined efforts of all the players that the story developed.
I've thought about this notion a bit more, especially as it relates to communal storytelling games.
I raised some ideas over on the Forge with regard to this (here).
I mentioned the notion of granularity in my post on the Forge, but I hadn't really formulated my ideas too clearly as I was writing the post. Now I've had some more time to think about it, and now that I've riffed a bit further off Jason Godesky's ideas, I present a more coherent notion for a currency of storyline element introduction.
Players would gain a fixed number of points at the start of play, maybe a dozen.
When introducing something, they need to account for three factors. The first is the potential risk to the group, the second is the difficulty to overcome the element, and the third is the duration of the event (how many scenes it takes to resolve).
A quick event takes only a single scene to resolve, a more complicated event takes a few scenes to overcome. For every scene of storyline impact, a player needs to spend points on the threat level and the difficulty level.
1 = Inconvenience
3 = Minor Injury
5 = Temporary Removal from the Story
7 = Permanent Removal from the Story
Difficulty to Overcome
1 = Easy to overcome (50% chance of success for most players, 75% chance of success for players specialised in this type of threat)
2 = Tricky to overcome (30% chance of success for most players, 60% chance of success for players specialised in this type of threat)
3 = Difficult to overcome (10% chance of success for most players, 30% chance of success for players specialised in this type of threat)
4 = Nearly impossible (5% chance of success for most players, 20% chance of success for players specialised in this type of threat)
Three different players might introduce seven point storyline effects into the game.
The first player introduces a threat that has a chance of producing minor injury (3pts), but is nearly impossible to avoid (4pts).
The second player introduces a threat that lasts for two scenes. In the first scene the characters are inconvenienced (1pt), but this is pretty easy to overcome (1pt). This sets the events in motion for a second scene where the characters have a chance of suffering a minor injury (3pts), in a way that is tricky to overcome (2pts).
The third player introduces something that has a chance of completely removing it's victims from the storyline for a short time (5pts), in a way that is tricky to overcome (2pts).
The numbers would require a bit of tweaking based on the game being played, but I'm sure you get the idea.
All players get to spend a fixed number of points to introduce these elements at the start of play, perhaps writing their plot elements onto index cards to be drawn randomly during the course of the game. One player could distribute their points across a few elements, while another player could consolidate their points in one huge whammy and a secondary minor element.
The GM would add their own variety of elements into the mix as well.
Experience could then be based on the total point value of the element encountered, but that would also require some modification based on the system into which the elements are placed.