You can simultaneously create story while relinquishing some GM control, injecting unpredictability in a slightly different way than the dice improv techniques I mentioned yesterday. You may have seen indie games like My Life With Master, with its stats that affect story development, or InSpectres, with a mission dice goal for each adventure (the mystery is solved when the players earn that many mission dice) or the InSpectres UnSpeakable variant, with its pool of Pit dice which indicate increasing tension as the pool shrinks. What tricks like this do is control the way a story develops without predefining events.
Assign hit points to a concept or plot event, as if it were a monster. Certain actions inflict "damage", and when that accrued damage exceeds the hit points of the concept or event, it happens; the plot changes.
A simple example: you start tracking an Envy stat while the party is in a particularly rough "thieves quarter". Every time NPCs witness the adventures displaying wealth ostentatiously, Envy takes 1d6 damage (perhaps a Wisdom save reduces damage.) When Envy has taken maximum damage, a thief has taken notice of the party and plots to steal the last item flagrantly displayed.
Or use tracker variables that rise and fall. Perhaps a Trollish Vengeance stat that begins at zero and increases by 1 every time a troll escapes an attack by the party; when the party returns to base, roll a d6 every day and have the trolls take vengeance on the party or someone perceived as an associate if the roll is less than Trollish Vengeance, then reset Trollish Vengeance to zero.
These are scenario-specific, but the concepts might actually be themes, like Demonic Influence, Betrayal, Ancient Blood Feuds. Pick the kinds of themes you want to see, start them all at zero. At regular intervals or under specific situations, roll 2d6: if you roll doubles and the value from 1 to 6 is less than the current Betrayal rating (for example,) an NPC betrays someone and Betrayal is reset to zero; otherwise, the theme tracker increases by 1. Select the victim at random; it doesn't have to be a PC, but the PCs must witness at least a part of it, and may hear about the rest as gossip. If they choose not to act, nothing more happens, but future events may reference this betrayal.
Themes are not meant to force characters to do anything; instead, they make stuff happen, fleshing out the setting and adding color, as well as giving the PCs something they could interact with, in any way they feel: do they side with one side or another when a blood feud erupts? Do they take advantage, playing both sides against each other? Do they rescue innocent bystanders? Do they flee, finding a new home base?
The more open-ended story trackers are played, the more satisfying the game will be. And in the next part of this series, I'll suggest something even more open-ended.