You can start by looking on fanfiction websites like Archive of Our Own or FanFiction.net. Use the search function and type in keywords like 'MCU crossover'. Check the ratings and reviews of the stories to see which ones are highly recommended.
One great example is a fanfiction that crosses over Iron Man and Doctor Strange. It combines their high - tech and magic elements in a really cool way. They team up to fight a new, powerful villain that neither could defeat alone. The story has a lot of action scenes and interesting character interactions.
One great 'X Men MCU Fanfiction' is 'The New Mutants in the MCU'. It's an exciting story that imagines how the young mutants would fit into the MCU. It has great character development and action scenes.
When watching the MCU fanfiction, it's a great idea to set aside dedicated time for it. This way you can fully immerse yourself in the stories. Try to find fanfictions that have been recommended by other MCU fans. Social media groups or forums are good places to look for such recommendations. Additionally, pay attention to the writing style of the authors. Some may have a more descriptive style that really brings the MCU world to life in their fanfiction, while others might focus more on dialogue and character interactions.
A Monsterverse MCU crossover fanfiction could involve the Titans from Monsterverse invading the MCU universe. For example, Godzilla could emerge in New York City, much like the Chitauri invasion in the first Avengers movie. The Avengers would then have to figure out how to deal with this colossal, irradiated beast. Maybe Iron Man tries to use his repulsor rays against Godzilla, but they have little effect.
One idea could be that a Sith Lord somehow gets transported to the MCU universe and tries to take over Hydra. They could use their dark side powers to manipulate the already corrupt organization. Another could be a story where a young Jedi in training from the Star Wars universe ends up in the MCU and teams up with the Avengers to fight a Sith who has also made their way there. Maybe they have to combine the Force with the superpowers of the Avengers to defeat the Sith.
Maybe the Flash could visit the MCU. He could race against Quicksilver. It would be a really cool showdown of speedsters from different universes. And then the Flash could get involved in an MCU - level threat, like Thanos' invasion but in a different timeline. He could use his speed to try and steal the Infinity Gauntlet from Thanos, creating a whole new plotline for a fanfiction.
In a MCU Monsterverse crossover fanfiction, perhaps the Infinity Stones could be involved. Let's say the Time Stone gets lost and ends up in the Monsterverse. Doctor Strange and some of the other Avengers follow it. There they find that the Titans are somehow connected to the power of the Infinity Stones. They need to work with the Titans to retrieve the Time Stone. Godzilla, being a powerful force of nature, could be the key to accessing the area where the Time Stone is hidden.
You can try Archive of Our Own (AO3). It has a large collection of fanfictions, including many MCU DC crossovers. Just use the search function and type in relevant keywords like 'MCU DC crossover' to find what you're looking for.
There's also a fanfiction where Thor and Superman are pitted against each other in a friendly competition that turns into a real battle when a powerful villain manipulates the situation. It shows how their powers compare and contrast. Thor's godly powers and Superman's alien abilities make for an interesting dynamic.