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.
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.
They contribute in several ways. Firstly, they keep the fans engaged between movie releases. They also introduce new concepts and ideas that might later be incorporated into the movies. For instance, some new villains or plot twists can first be tested in the graphic novels. Also, they can provide different interpretations of the characters, adding more dimensions to the overall MCU franchise. Moreover, they can attract new fans who are more into reading graphic novels rather than just watching movies.
Not always. While the MCU draws inspiration from the comics, there are often changes and adaptations made to fit the cinematic universe and modern audiences.