Well, here’s my ideal …
Before we start, I provide the CONTEXT for the scene, whether we’re looking at a kingdom or a tavern, or an event like a combat, or the Big World Map. THIS is a very generalized map, THESE are some important bits of history or theme or function or widely-known facts, aaaaaaaaaand … I’m done. Nothing I write about anything should ever be more than a paragraph.
As part of their backstories, the players provide all the DETAILS about their character’s homes, whether it’s a kingdom, a town, or whatever. They can provide as much or as little detail as the like—maps, people, customs, history, etc.—as long as it jives with my very broad context (or at least contradicts it in an interesting and meaningful way that I can build a hook on).
This by itself shouldn’t require any special game mechanic—but I would like to have a mechanic that keeps that backstory popping up throughout the game. It’s one thing for the GM to repeatedly cross your path with an old childhood rival, but I’d like a mechanism by which the players can say “I have an old childhood rival. I think it would be good for the story if he showed up this session.” This way they can volunteer for their own conflict, and earn XP or whatever accordingly … and because it’s their own idea and not a “gift” from the GM, I know it’s something they’re going to have fun with.
So that’s one mechanic I’m hoping to see. Here’s the other:
Paranoia has a mechanic where, in combat, the GM doesn’t actually tell the players anything about the scene. You’d think this would make it hard to get cover bonuses and all that, but the players actually get to choose their OWN bonuses by describing the scene to the GM: “I really want to hurt this guy, so let’s say there’s a bunch of pipes right next to him, and I shoot the pipes and they burst, dousing him in radioactive acid that’s on fire?” Could there be pipes there? There’s a point-buy system to keep it a game, but SURE!! +15 to damage! Or “I really don’t want this guy to hit me, so let’s say his laser rifle jams when he pulls the trigger.” Spend points … SURE!! +5 to your Evasion or whatever.
This particular mechanic would only work in Paranoia, but something similar that lets the players say “We’re in a marketplace, right? So there’s lots of vendors around, and probably a bunch of wine barrels stacked somewhere? I kick the barrels over and roll them down the street at the guys chasing us!” Were there barrels there in the first place? I never said there WASN’T, and it does fit the context of a city bazaar … you probably just never noticed them until you realized they might be useful, but sure, they were there the whole time.
Other situations might be when the story dead-ends and one of the characters suddenly “remembers” something that helps get the game moving again. When the GM does this, we call it “deus ex machina” and it’s lame, but when the players get to do it, they feel like they’re in charge and it’s fun.
Often in Savage Worlds I’d let players spend a benny to take narrative control like this, even though it’s not in the rules. Other times they just do it and I let it go because it’s cool and they’re getting more involved. But I think a mechanic that actually encourages this behavior would be the cat’s pajamas. So whether that’s a moment-to-moment die roll kind of thing, or a GM “passing the torch” for a scene … but as long as there’s a game reward for the player investing their creativity. Because that’s what encourages them invest their creativity again … and again … and again …