A crossover with Doctor Strange could be really cool. Maybe they have to deal with a magical threat that is spreading across different dimensions. Captain Marvel's strength and Doctor Strange's magic would create an interesting dynamic as they try to figure out how to stop it.
One idea could be a story where Captain America and Captain Marvel team up to face a new cosmic threat that's headed towards Earth. They have to combine their unique skills, with Captain America's strategic mind and combat experience on the ground, and Captain Marvel's incredible cosmic powers in space battles.
Fanfiction.net also has a lot of fan - made stories. There are some really creative writers on there who write Captain Marvel crossovers. Just be aware that the quality can vary, so you might need to do a bit of digging to find the good ones.
There could be lots of possibilities. Like exploring different timelines and powers. But a challenge might be keeping the characters' personalities true to the originals.
One great fanfiction is 'Allies in the Cosmos'. It beautifully weaves together the storylines of Captain America and Captain Marvel. In this fic, they team up to face a new alien threat that neither could handle alone. The writer does an excellent job of capturing their unique personalities and how they complement each other in battle.
One great idea could be a story where Percy Jackson's world is in danger and Captain Marvel is somehow drawn into it. Maybe a powerful ancient Greek monster has powers that start to affect other dimensions, and Captain Marvel comes to help. She could use her super strength and energy blasts to fight alongside Percy and his friends against the monster. Percy, with his control over water, could also be useful in a fight, like creating a giant wave to distract the enemy while Captain Marvel attacks.
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.
Such fanfiction is not okay. Captain Marvel is a powerful and inspiring character. Creating sexual fanfiction about her reduces her to a mere object of sexual fantasy rather than celebrating her as a hero. It also may make many fans uncomfortable, especially those who look up to the character for her strength and positive influence.
Captain Marvel is Carol Danvers. She gained her powers through an interaction with alien technology and has become a powerful hero in the Marvel universe.
They could have a mentor - mentee relationship. Batman, with his vast experience, could mentor Captain Marvel on things like stealth and detective work, which are Batman's specialties.