In most cases, it is legal. Fanfiction is a form of creative expression by fans. As long as it doesn't violate any copyright laws in a major way, like using the exact same footage from the trailer without permission in a way that undermines the original work's marketability. But if it's on a legitimate platform that respects intellectual property, you should be okay.
Well, sometimes YouTube channels dedicated to fan content might feature or recommend Infinity War trailer fanfiction in video form. However, make sure it complies with YouTube's terms of use. Also, some specific fan communities on social media platforms like Reddit might share links or discuss where to find such fanfiction.
There could be a fanfiction where a group of friends, who are all huge Marvel fans, watch the Infinity War trailer together. They then start speculating about the possible plot twists and character developments. In the fanfiction, they might have a detailed discussion about how Thanos' character will be portrayed and what could be the fate of their favorite heroes like Iron Man or Captain America. It could also include their wild theories about new characters that might be introduced.
You can try platforms like YouTube. Sometimes fans create and post fanfiction - inspired videos related to the Avengers Infinity War trailer on there. However, make sure to abide by the platform's terms and conditions.
The popularity of Avengers Infinity War trailer fanfiction lies in the huge fan base of the Avengers franchise. Fans are so passionate about the characters and the story that they want to create their own versions based on the trailer. It gives them a chance to explore different plotlines and character developments that they hope to see or imagine in the movie.
A fanfiction idea could be a time - traveler who goes back in time after seeing the Infinity War trailer. Their goal is to warn the Avengers about what's coming and try to change the course of events. They face many challenges as they try to convince the heroes that they are from the future and that the trailer they saw is a real preview of a disastrous future.