One interesting storyline could be about the characters facing a new, powerful dark force that they have to combine their powers to defeat. For example, a shadowy entity that can manipulate time, making their usual strategies ineffective.
Maybe a fanfiction where Yui loses her memories and Hinata has to help her remember who she is and all the things they've been through together. He could show her the places that were important to them, like the concert stage where Yui used to perform, and slowly her memories start to come back as their relationship deepens.
There could also be a fanfiction where Percy has to face his inner demons in a more psychological way. Maybe he's haunted by the memories of all the battles he's fought and the friends he's lost. His friends then have to help him through this tough time, and in the process, they learn more about each other's deeper fears and insecurities.
Another possible storyline is that they become friends after an accident. Maybe during a Quidditch match, there's a terrible accident and they have to rely on each other to survive. This shared experience makes them see each other in a new light and they start to build a friendship. They then help each other deal with their respective family issues. Draco can give Harry insights into the pure - blood families' ways, and Harry can show Draco a different way of looking at the world, not just through the pure - blood ideology.
There could also be a storyline where Naruto and the Kyuubi team up to face a common enemy that neither of them could defeat alone. They have to rely on each other's strengths, with Naruto's determination and the Kyuubi's raw power. This adventure would bring them closer as friends and show the world that they are a force to be reckoned with.
One idea could be exploring the backstories of the angel friends in more detail. For example, you could write about how they first became angels and what their lives were like before joining the angelic realm. Another idea is to create a new adventure for them, like a quest to save a part of the celestial world that is in danger. Maybe they have to face a new kind of enemy or overcome a powerful curse.
One interesting storyline could be them teaming up against a new supervillain that neither can defeat alone. They combine their unique skills, Superman's super strength and flight with Batman's detective skills and gadgets.
In Lego Friends Stories, there might be a story where the friends go on an adventure to find a lost treasure. They use their different skills. For example, one might be good at reading maps, another at climbing. Along the way, they make new friends and also have to outwit some sneaky characters who also want the treasure. It's a story full of excitement and friendship.
One possible storyline could be that Harry discovers his angel wings during a dangerous battle at Hogwarts. The wings give him the ability to fly faster and more gracefully than a broomstick, allowing him to outmaneuver the dark forces. He could use them to rescue his friends from precarious situations, like flying them to safety when the castle is under siege.
One interesting storyline could be them uncovering a long - hidden family secret. Maybe there was a third sibling they didn't know about, and they go on a journey to find this mystery person. Another could be them dealing with a threat to their family that only they, as siblings with their unique abilities, can face.
In some fanfictions, the friends' letters in OotP could be a way to introduce new magical elements. For instance, a letter might be sent on a special parchment that has the ability to transform into a small creature to deliver the message more quickly. Or perhaps the letters are charmed in such a way that when they are read, they create a temporary magical scene related to the sender's current situation, adding a new dimension to their communication.