A quest type story where only the MC knows the goal of the quest. The MC must convince others to help without revealing the goal, must make it through many trials/challenges, and successfully complete the goal without it being revealed to anyone (even the reader) until the quest is completed.
I think this type of story would be very challenging for an author to write, especially if longer than a short story as it would require very skillful writing to engage the readers' interest without revealing too many details that might give away the goal of the MC.
It lends itself well to shorter stories where it could describe a scavenger type hunt, a no-say/no-tell condition of a school kid's challenge, a vacation taboo, etc... but for a longer story with an actual quest there would need to be a rationale for keeping the goal secret, lots of plot twists as well as incidences designed to expose the goal, etc... and these would have to written in such a way to give no hint to the reader as to what is the actual goal.
Anyone up for the challenge?