Well, first you need a great idea. Then, do lots of research to make your story authentic. Plan out your plot and characters carefully. Start writing and don't be afraid to rewrite and edit.
Jenny and Tyler lived in a small seaside town. One stormy night, a shipwreck occurred near the shore. They heard the cries of the sailors and rushed to help. They found a young sailor who told them about a map he was carrying that led to a lost island full of wonders. Jenny and Tyler decided to embark on a journey to the lost island. They sailed for days and finally reached the island. There, they discovered beautiful beaches, strange animals, and a secret cave filled with gold coins. They shared the gold with the sailors and became heroes in their town.
Once upon a time in the spaceship of Among Us. The crewmates were going about their tasks as usual. There was a green crewmate named Lime. Lime was very diligent and always finished tasks quickly. One day, while doing the wiring task in Electrical, Lime noticed something strange. The lights flickered not because of the task but as if there was some unseen force. Lime became suspicious and started to look around carefully. Suddenly, a red imposter appeared. Lime quickly ran towards the emergency button but was caught by the imposter just before reaching it. And that was the end of poor Lime.
One idea could be to rewrite the story from a different character's perspective. For example, if there is a side character in 'the story of us', you can show their thoughts and feelings throughout the events. Another idea is to create an alternate ending. Maybe in the original, the couple gets together, but in the fanfiction, they part ways for some unforeseen reason. You could also add new characters to the story that bring in new conflicts or relationships.
One idea could be exploring a romantic encounter between two characters in an unexpected location, like a hidden beach on their family vacation. Maybe they share a deep conversation and a passionate kiss under the moonlight.
One idea could be exploring what would happen if Lily and Ryle had met in a different context, like in a different city. Another could be writing from Atlas's perspective during his time away from Lily, detailing his struggles and growth. Maybe also a fanfic where Lily's mother had a different influence on her relationship choices.
One interesting idea could be exploring the backstory of a minor character. For example, you could write about how a random survivor in one of the quarantine zones came to be there. Another idea is to create an alternate ending. Maybe Joel and Ellie find a different way to deal with the Fireflies that doesn't involve so much moral ambiguity. A third idea might be to set the fanfic in a different location. Instead of the usual post - apocalyptic cities, it could be in a more rural area and how the infected and survivors interact there.
One idea could be exploring the backstory of a minor character in the game. For example, you could write about how a random survivor in one of the factions got to be where they are. What events led them to join the group? What were their hopes and fears before the apocalypse? Another idea is to create an alternate ending. Maybe Ellie decides not to go on her revenge journey and instead focuses on building a new community. This could change the whole dynamic of the story.
One idea could be a crossover where the magic from Harry Potter is used to combat the infected in The Last of Us world. For example, the Patronus charm could be used to repel the Clickers as it is a powerful form of positive magic. Another could be that some of the characters from The Last of Us are transported to the Hogwarts world and have to learn magic to survive in this new and strange place. They could be sorted into houses based on their personalities.