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.
MCU sex fan fiction is not something that is officially recognized or endorsed by Marvel Studios. Fan fiction in general can be a creative outlet for fans to explore different scenarios, but when it involves sexual content, it can be inappropriate and may make some fans uncomfortable. Many fans prefer to focus on the positive aspects of the MCU, such as the action, the character development, and the overarching storylines.
In my opinion, it's not acceptable. The Marvel Cinematic Universe has a certain brand image that is centered around heroism, adventure, and positive values. Sex fan fiction undermines this by introducing inappropriate and often out - of - character sexual elements. It can also make the fan community seem less friendly and more seedy, which is not what most fans or Marvel itself would want. Additionally, it may face legal issues regarding the use of copyrighted characters in an improper way.
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.