It could be a powerful alien warlord who has set his sights on Earth. This antagonist might have an army of powerful minions and technology that rivals Captain Marvel's and the Avengers' combined powers.
Maybe it would be a rogue Asgardian who has been banished and wants revenge on Thor and the Avengers. He teams up with some dark forces from other realms and becomes a major threat when Captain Marvel enters the picture. He could use his knowledge of Asgardian magic and the new alliances to create chaos.
In this fanfiction, the main antagonist could be an interdimensional being. This being has the ability to cross different dimensions and is draining the energy from our universe to power its own. It sees Captain Marvel and the Avengers as the only ones who can stop it, so it starts attacking them preemptively. It has unique powers that the heroes have never faced before, like the ability to nullify certain types of powers in specific areas, which makes the fight very challenging for Captain Marvel and the Avengers.
In this fanfic, the main antagonist could be a scientist who has been corrupted by his own experiments. He creates some sort of energy - based monsters that are immune to normal attacks. Captain Marvel and the Avengers have to figure out how to stop these creatures and the mad scientist behind them.
Thanos could still be a great antagonist even in a fanfic. He could come back with a new plan to obtain the Infinity Stones again. His powerful and menacing presence would create a high - stakes situation for Captain Marvel and the Avengers to face.
Well, perhaps in the 'Captain Marvel meets the Avengers' fanfiction, there would be some interesting character dynamics. Captain Marvel, being from a different part of the universe, could introduce new concepts and technologies to the Avengers. There could be a bit of a culture clash at first, like her more direct and no - nonsense approach might not sit well with some of the more laid - back Avengers. But eventually, they would learn to work together and combine their unique skills to save the day.
Another possibility is that Death is being controlled or influenced by a greater cosmic entity. In this case, that entity could be the main antagonist. It uses Death as a tool to test or destroy the Avengers. The Avengers would then have to not only deal with Death but also find a way to face this more powerful and mysterious entity behind Death's actions.
There could be a storyline where Captain Marvel meets the Avengers during a time - travel adventure. She might be from a different time - line herself and they have to figure out how to fit her into their current reality. Or perhaps there's a story where she has a secret mission that the Avengers accidentally get involved in, and they end up working together to complete it.
When Captain Marvel meets the Avengers in fanfiction, it could lead to some amazing adventures. Imagine her leading the Avengers on a mission to a far - off planet. She could use her flight and energy - projection abilities to get them there safely. Once there, they could face off against some new and terrifying enemies. Her knowledge of the cosmos could give the Avengers an edge in battles and diplomatic situations. And she could also form a special bond with Black Widow, sharing stories of their tough pasts and how they overcame them.
Maybe a group of bounty hunters hired by the remaining Imperial forces. They see Luke as a big threat and Captain Rex as an annoyance. They keep hunting them down throughout the story, creating all sorts of dangerous situations for our heroes.
Well, in a lot of 'Avengers Captain Marvel fanfiction', Loki can be a very interesting character. His mischievous nature can lead to all sorts of plot twists when interacting with Captain Marvel. Scarlet Witch is also popular, as her powers and Captain Marvel's can have some really cool and unexpected interactions, whether they are on the same side or opposing each other.
Captain Marvel and Thor make an interesting pairing. Their powers are both related to cosmic elements, with Captain Marvel having her energy - based abilities and Thor his godly powers. In a fanfiction, they could team up to battle some inter - galactic villains and their combined strength would be quite a sight.
Annihilus could be a likely villain. He is a powerful being from the Negative Zone. In the fanfiction, Annihilus could be drawn to Earth because of the energy signature created when the Avengers and Captain Marvel meet. He sees Earth as a prime target for conquest. His powerful cosmic control rod gives him an edge in battle. The Avengers and Captain Marvel have to quickly learn about his weaknesses and use their combined powers to stop him from destroying Earth and spreading his reign of terror across the universe.