One interesting storyline could be a new hero emerging from a small town. This hero has a unique power that the Avengers initially mistake for a threat. So, there's a lot of tension and action as the Avengers try to figure out if this person is friend or foe. Eventually, they discover the hero's good intentions and they all team up to face a greater threat like an alien invasion.
Another could be a story about an ordinary person who gets accidentally caught in an Avengers battle. They end up getting some of the Avengers' powers due to an experimental weapon that goes haywire. This person then has to learn how to control these powers while also dealing with the Avengers who are a bit suspicious of them at first but later come to accept and train them.
One interesting storyline could be the students at Avengers University having to compete in a series of super - powered challenges. They might have to use their unique abilities, whether it's super strength, telepathy or the ability to fly, to overcome various obstacles. This would not only test their skills but also their teamwork and strategic thinking.
There are also storylines where Spiderman's secret identity is at risk within the Avengers. Maybe a villain has found a way to infiltrate the team and is trying to expose him. This creates a lot of tension and drama as Spiderman has to protect his identity while still being part of the team.
One interesting storyline could be the Avengers having to deal with threats on the homefront that are more personal and civilian - based. For example, they might have to protect a small town from a new type of villain who targets local infrastructure. Another could be the Avengers trying to integrate into normal life while still being superheroes, like Steve Rogers getting a regular job in addition to his superhero duties.
A lot of fanfiction focuses on Loki's redemption arc within the context of the Avengers. For example, Loki could be injured and the Avengers take him in instead of imprisoning him. This leads to him slowly seeing the value in their cause and starting to change his ways. He might start using his powers for good, like helping Thor understand some new magical threats, which also helps to strengthen the bond between him and the Avengers.
One interesting storyline could be the Doctor using his time - traveling abilities to visit different eras in the Avengers' timeline and help them prevent disasters that they otherwise couldn't. For example, he could go back to the events of the first Avengers movie and use his knowledge to stop Loki's plan more efficiently.
One interesting storyline could be exploring the backstories of the minor characters in the Masters of the Universe universe. For example, delving deeper into the origin of Orko, his life on his home planet before coming to Eternia. Another could be a story where He - Man and Skeletor have to team up against a greater evil that threatens both their interests. This would flip their usual dynamic on its head.
One interesting storyline could be about an alternate version of Sam and Dean where they are vampires instead of hunters. It would completely flip their roles and moral compasses.
One interesting storyline could be She - Hulk joining the Avengers on a mission to a parallel universe. In this universe, the laws of physics are different, and they have to adapt quickly. She - Hulk's strength and legal expertise could come in handy when dealing with the new and strange situations there. Another could be a story where She - Hulk has to defend an Avenger in a court of law within the superhero world, which would be a really unique take on the superhero genre.
One interesting storyline could be Spider - Man having to use his unique abilities to solve a mystery within the Avengers' headquarters. His agility and spider - sense might lead him to discover a hidden threat that the other Avengers overlooked.
One interesting storyline could be a crossover where Agents of Shield agents have to team up with the Avengers to stop a powerful alien threat that neither group could handle alone. This would involve a lot of character interactions, like Coulson trying to give orders to Iron Man or Natasha Romanoff teaching some of the Shield agents her spy skills.