My favorite type of fanfiction involves the act of changing one or two small details that change EVERYTHING about the characters and how they interact. The best truly try to archive these alternate realities with as much integrity to the source material as they can. It's a wonderful mental exercise and I truly believe that young people echo participate in this exercise, be it about Master Chief or just straight slashfiction, will grow greatly in doing so.
You know what's coming next. An example. Here we go. Let's try to create a new universe out of the Avatar universe. What if we just swapped air with fire? What if the fire benders were the peaceful, nomadic benders and the air benders were the army republic? "Everything changed when the air benders attacked." This begs the questions then: how are the air benders to match the destructive force that the fire benders were in the show and how are the fire benders going to make sense as pacifist nomads?
Air benders could be brutal if they were not pacifists. Air benders could be flying dervishes of lethal pressure blades, they could launch others sky high and let gravity take care of business with all the ruthless efficiency of physics, an air bender could even suffocate their foes if they simply did not let them breathe. Good villains? Check, but we will have to modify the respective characters to fit their new world. Perhaps Zuko's father severed his hand instead of scarred his face. How would this change his attitude and his legend?
In the third book, Aang learns that fire does not need to be about rage and destruction, but instead can be seen as the primal fire of the heart-- the spark of life that animates the clay of our bodies. Perhaps fire benders in this fic practice pacifism to align their lives towards this belief. Their flames would shine like the heavens as they defended. A touch could draw that life force away to stun or even to radiate to revive and revitalize. Pursued far enough, I believe the flame of a fire bender could solely sustain the human body, allowing these nomadic healers to never need to eat (they'd drink a lot though and need to pee all the time). Aang's whole journey and struggle for nonviolence remains intact with this small detail switched, but all the details change. Who teaches Aang the gentler side of the ruthless air bending? What kind of children would he and Katara had? How would this new historical legacy of fire mesh with the existing spiritual mythos of the story?
These questions and the details they reveal are the keys that make this exercise so powerful. Every question is an act of creation. You take a young and imaginative mind and just start getting them to ask these questions and explore in the endless universe fountain of their imagination about whatever it is that interests them. Be it a book or a movie, a video game or a garden, this exercise is not only fun, but also intense training in a skill that is absolutely necessary in the dire straights my generation is in.
So yes. That, dear readers, is why fanfiction is the national pastime of my brain. Have a nice day with that.