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.
A popular one is 'Heroes United'. It's a more light - hearted take on the crossover. Flash and Quicksilver have a friendly race that turns into a mission to stop a time - traveling villain. It's full of humor and great action sequences that show off the speedsters' abilities.
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.
One common trope is the 'fish out of water' where the X - Men characters are suddenly in the MCU and have to adapt to the different rules and characters. For example, in many fanfictions, Wolverine has to deal with the high - tech world of the Avengers.
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.
You should start by creating a concept. Maybe it's a world where the Infinity Stones and the Mother Boxes exist together. Next, plan out how the characters will interact. Will they be friends or foes at first? For instance, if you're crossing Wonder Woman and Thor, think about how their different cultures would play into their relationship. Then just start writing, and don't be afraid to let your creativity flow.