So, I've been GMing an RPG set in space recently. The group took the first part of the campaign to escape from an interstellar prison-planet (yes, it's Butcher Bay. I know). Some characters died during a prison-riot that the group had started in order to form a distraction, but most of the group succeeded in escaping, forming bonds in the process, being cool and all.
Playing a campaign in prison had some advantages for me as a game master. The environment was limited, so were the NPCs to interact with - I made an exhaustive list of them and how they were grouped and what they were like. I had no plan how the group would escape the prison. They would have to figure one out themselves. It was a small-scale sandbox for my group to play in. Now, for what we call our second season, we have some new characters, some old ones, and the whole thing is set on a small ship escaping from the law. We basically went from Butcher Bay to Firefly. Which is nice. But entirely different in structure.
Originally, I had a group of players in a very controlled environment, trying to survive the other inmates whilst planning an escape below the radar of the omnipresent guards. Now there is no law, as the group does well to be always a step out of range of any civilized law-enforcement. The only true danger comes from bounty-hunters and whatever criminals they deal with in order to make ends meet. And the world has grown infinitely: My map as a GM in the first campaign was a map of the prison-complex. Now it's a star-map.
As I often do, I went at this without too much of a plan for what the players would do in the world was creating. Most of the player characters had enough baggage when they came into prison - they had unfinished business on the outside and the interesting bit would just be whose business they would go at first and if they'd stick together as a group. That, to me as the GM, was the greatest threat: The group falling apart, each of them going their way after escaping. So it was a question of who would have the most sway in the group.
So now they are playing a campaign based on the background-story that one of my players made up for her character. Trying to find the slavers who sold her as a child, in order to find out where she's really from. And the rest of them is on board with this. In a GM-lead game we have a co-created storyline where the inter-player dynamics are the driving force and the world gets broadened wherever it is needed. It is glorious.