By mystery I mean who, what, where, when, why - where the players start with zero knowledge. Is the noble a friend or a foe. Can the guard captain be trusted. Who is responsible for the abductions one the docks. Where does the strange black door lead to. What is the purpose of the small vials of brain matter found in the murdered merchant's belonging etc etc. These are things I consider mysteries. Its also most of the exploration tier of play, which people in D&D terms sometimes think is all about rolling survival checks. When in truth its about exploring the unknown.Decreases mystery how? This seems related to the point I was making. There can be mystery in the game world. There shouldn't be mystery for the players in understanding how the game works.
I'm saying that players deciding these things can lead to self-indulgence. The DM on the other hand is there to provide entertainment to the players. Five people creating a story can lead to a fractured, unconnected and self-indulgent elements. I see it all the time when multiple authors work on Adventure Paths. I think it leads to a weaker product.So, you're saying that allowing multiple participants to decide stakes is self indulgent... but having one participant do so isn't? Doesn't that seem inconsistent?
You can do both. A story can be revealed but also influenced and changed by the players. As soon as @ZebraDruid decided that the noble wanted to kill his family he started to tell a story. Agency is about being able to influence the story. Often I see people equate agency with autonomy - being able to act independently of influence or oversight. Agency is great - autonomy isn't practical for the kind of games I like to both play in and DM.Regarding the challenge... I'm not saying it can't be fixed beforehand. I'm saying that information should be made available to the players.
Are you telling a story or playing a game? I think that's probably a key question to ask when we start playing. If we're concerned about player agency (and we don't have to be, it's a preference), then we have to focus on the player... we have to focus on the game.
If you're telling a story, then player agency isn't going to be as much of a concern.