In tremulus, a "storytelling game of lovecraftian horror", there exists the concept of framework, a slightly formalized way of describing key points of a scenario. One of the elements of the framework is "the Unknown: open questions to be answered during play." These are major questions significantly influencing the story and the mystery, though they do not define it—other elements of the framework define the overall threat, how it progresses and what happens when it succeeds. One of the examples provided in the rulebook:
How powerful is the mayor? Does he already know the dark ritual? What exactly are the Creatures in the Woods? What are their powers?
However, there doesn't seem to exist a method for collaboratively answering these open questions with the input of players. The game suggests asking provocative questions of the players, but all the examples provided are limited to getting them involved in exploring the mystery:
Certainly, they can see a figure in the graveyard, but they can’t make out who they are or what they’re doing. Do they get closer?
Further, on the official forums the game creator answer a question about dispensing information with Lore or Clues that players earn:
You can certainly collaborate, if you and your group are willing to have a little give-and-take. Just be certain to keep things in the proper tone, and atmosphere, and not let them enter ridiculous areas. It's best to provide a little lead and direction, such as "You notice something about the painting that's unusual... what do you think it could be?"
tremulus is based on the Apocalypse World engine, which I am unfamiliar with. I understand there is a Maelstrom in AW, which is always undefined at the start, for players to answer questions about and thus define as they interact with it.
Are there some elements of rules in AW that are not explained for whatever reason in the Kickstarter (unfinished) edition of the tremulus rulebook, that would allow for player-driven answers to the Unknown? A major part of tremulus is exploration of mysteries, and these answers would have to fit the other parts of the framework.