I feel like some past games I've helped run have had some luck with encouraging player run plots. I think a few things have helped.
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A theme in which the space for relatively stand-alone plots consistently presents itself. I've run.. gosh, three games now. Where the characters did jobs, and those jobs were short plots. The ability to run a plot that is limited to 2-3 scenes is REALLY helpful for new GMs, or even just people who don't have the time and energy to commit to something that might spiral into long and complicated. It's lower effort, lower risk. This structure also really helps with ideas, as it presents a straightforward framework for the sorts of plots that fit easily into the game AND a straightforward reason for most, if not all, characters to participate. It's PRP'ing on easy mode, which I think is super important for giving people confidence, especially if they've never GMed before.
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A culture where PRPs are regularly and clearly asked for. Not just encouraged in a theme file, but a post saying 'hey, is anyone willing to run something this month?' Sometimes even saying to a particular person 'would you be willing to run a scene?' Sometimes people need to be asked.
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A clear method for running plots. Do you have to take everyone who wants to come, or can you limit numbers? How do you resolve conflict? What rights do you have as a player GM to make things up? How are off-camera actions handled? What needs to be RPed vs not RPed? I think a clear game philosophy to GMing helps with this, too. Does your game trend toward letting players succeed, or do you rely on dice, even when they're brutal? Is your system the final decision maker, or can GMs say 'no, you aren't really knocked out, let me undo that, it was stupid'?
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A game commitment to supporting new GMs. One of the best things for this, in my opinion, is to team up for a PRP. An experienced GM, or even staff member, and someone who's new to it. Brainstorm together, outline together, run scenes together. I've run a LOT of plots, and teaming up is still my #1 favorite way to do it.
We've tried other things, like idea banks and story ideas, and they have been minorly helpful, but I think those things are mostly only helpful when you already have culture aimed at supporting and developing GMing ability and confidence in people.