A plot could be about an alternate universe where Captain Marvel is initially distrusted by the Avengers. She has to prove herself and in the process, they all learn more about each other's powers and pasts. This could lead to some great character development and new alliances within the team.
One idea could be Captain Marvel bringing back some alien tech that the Avengers have to figure out how to use. It could lead to new powers for some of the members or a new way to defend Earth.
Imagine a fanfiction where Captain Marvel uses her energy - based powers in a completely new way during the final battle in Avengers: Endgame. Instead of just punching and flying around, she could manipulate the energy fields in a way that disrupts Thanos's entire army. This would be a creative take on her character and her abilities.
One idea could be a time - travel fanfic where Captain Marvel and Captain America team up in a different era. Maybe they end up in the 1920s and have to deal with a Hydra - like threat that's starting to form early. Another could be a fanfic where they swap powers for a day and have to learn to adapt to each other's abilities. For example, Captain America trying to control Captain Marvel's photon blasts and Captain Marvel having to rely on just her combat skills like Cap usually does.
One possible plot could be that the Avengers are facing an extremely powerful new enemy that they can't seem to defeat. Captain Marvel arrives on Earth just in time. At first, there are some misunderstandings and clashes of egos among the Avengers and Captain Marvel. But as they start to work together, they realize each other's strengths. Captain Marvel's raw power combined with the Avengers' teamwork strategies slowly turn the tide of the battle. Eventually, they defeat the enemy and Captain Marvel decides to stay on Earth for a while to help the Avengers train for future threats.
One idea could be that she absorbs an unknown energy source which gives her new powers but also makes her a target. Enemies might try to steal or control this new power within her.
One idea could be Captain Marvel arriving just in time during the final battle in Infinity War. She could use her powers to turn the tide against Thanos. For example, she might team up with Doctor Strange to create a powerful energy shield to protect the remaining Avengers while they figure out a way to remove the Infinity Stones from Thanos' gauntlet.
Another concept could be a time - travel fanfic. Black Widow somehow gets sent to a time when Captain Marvel was just starting out as a hero. Black Widow then has to train the younger Captain Marvel and help her unlock her full potential while also trying to find a way back to her own time.
One idea could be a story where Captain Marvel and Valkyrie team up to fight a new, powerful enemy that threatens the entire universe. They start off not getting along due to their different fighting styles and personalities, but as they face challenges together, they learn to respect and rely on each other.
One idea could be exploring what if Loki had an unexpected ally within the Avengers. Maybe a character who was initially on the heroes' side but got swayed by Loki's charm or promises.
One plot idea could be that the Inhumans' city of Attilan is in danger and the Avengers are called in to help. The Inhumans are initially hesitant to accept their aid due to their differences, but they soon realize they need to work together. Another idea is a young Inhuman with a powerful but uncontrollable ability is found by the Avengers. They try to help the Inhuman learn to control it while also protecting them from those who would use their power for evil.