One possibility in Captain Marvel x Valkyrie fanfiction could be an adventure where they team up to fight a common enemy. They both are strong and powerful characters. Maybe they start off not getting along well due to their different personalities and ways of doing things, but as the story progresses, they learn to respect and rely on each other.
Another fanfiction idea could be a prequel to her role in Avengers: Endgame. It could show what she was doing in the cosmos just before she received the call to return to Earth. Maybe she was facing a different threat that was related to Thanos in some way, and this backstory could add more depth to her character in the movie.
One possible fanfiction could involve Spider - Man and Captain Marvel teaming up during the Endgame battle. Maybe Spider - Man uses his agility to distract the enemy while Captain Marvel blasts them with her powerful energy. They could also have some fun banter, like Spider - Man making a joke about how Captain Marvel can fly and he has to swing around.
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.