Just like the title says.
So I've had this idea and executed it a few times. I've seen it both lead to a daring adventure, and lead to completely ridiculous sandbox, and I've had fun in both. So essentially, the setting takes place at a location called the World Engine. In it, characters can try to achieve one of their wishes (or just mess around) by creating worlds in which the their goal will reside in. The main base on which the world will be built upon is usually a power memory of the creator's past, albeit a very corrupted version of it. There's a scale in which players place figurines to influence what is created. One side is positive effects, the other is negative, but the scale must remain balanced. In theory, the more positive figurines on the scale, the more tools the players have to accomplish their objective. Otherwise if there aren't any figurines the task would be impossible. Worlds also have a limited lifespan, so you can't stay forever. The more figurines. the shorter the lifespan. Time can be increased by placing hourglass figurines on the scale, but, those too need to be balanced. So not having enough figurines can make the task difficult, but too many can lead to a strict time limit. All of this is usually explained step by step by the Warden, an NPC who oversees the operation, over a number of posts, so there isn't any kind of info dump like I'm doing here.
One "serious" example is when one player had a ribbon that was very important to them. It reminded them of their wife or something. Anyway another character thought it'd be a great idea to place a figurine of that ribbon on the negative side of the scale. When the group entered the world, due to some other combinations, death row inmates were given a cursed version of the ribbon (they couldn't take it off) that marked them for death. The party had actually entered during a public execution. (It was a more mature website. Stuff like this could fly.) The one character who originally had the ribbon got pretty upset, and revealed his own ribbon condemning the crowd for their actions. This... led to a mob immediately trying to subdue the group. This quickly lead them to retreat back through the portal and rethink their plan.
On the more "fun" example, another group decided to mess around rather than chase any goal. So they kind of wildcarded the base of the world, and placed a weird combination of figurines. In the end, there were anthropomorphic pancake men being chased by syrup bottles in order to create a complete breakfast. For some reason, the group also decided to kidnap one of the pancake men and interrogate it for... reasons. I can't quite remember. The group was an odd bunch.
So, any opinions on the matter?