Definitely the Avengers as a whole. However, some fanfictions might also put a spotlight on Jarvis or Vision. Jarvis was important in the lead - up to Ultron's creation, and Vision was a key part of the solution to the Ultron problem. So, their roles could be expanded and their characters more fully developed in the fanfiction compared to the movie.
The main characters are the Avengers. This includes Iron Man, Captain America, Thor, Hulk, Black Widow and Hawkeye. Ultron is also a major character as he is the main antagonist in the story.
Black Widow is also a great character to focus on. Her past as a spy and her relationship with Hulk are interesting aspects. Fanfiction could explore her feelings towards her teammates and how she copes with the new threats in 'Avengers Age of Ultron'. Maybe a story about her training the new recruits or something related to her secret agent skills.
There's a chance that side characters get more focus too. Maybe characters like Rhodey or Nebula could have more in - depth character development as they react to the events of Endgame while being watched by the Avengers in the fanfiction.
It can also be frustrating at times. Some fanfictions lack the proper understanding of the movie's lore. They might introduce concepts that don't fit well with the established Marvel Cinematic Universe. But on the positive side, many fanfictions are a labor of love. Authors pour their passion for the movie into their writing, and that can really shine through, especially when they capture the essence of the characters as we know them from the movie.
Well, one reaction could be excitement. Fanfictions often explore different aspects of the characters and storylines that might not have been fully delved into in the movie. For example, they might focus on the relationships between the Avengers in more depth, like the budding romance between Black Widow and Hulk. It can be really interesting to see how different authors interpret the characters' emotions and motives.
Captain America is also a key focus. His leadership qualities are highlighted. He could be seen rallying the other Avengers as they watch the trailer, giving inspiring speeches about not giving up and fighting for the universe.
You can try looking on websites dedicated to fanfiction. For example, Fanfiction.net. Just go to the site, use their search function and type in 'Avengers Age of Ultron'. It'll show you a bunch of fan - created stories. AO3 is another good place. They have a lot of high - quality fanfiction, and it's easy to navigate through their categories to find what you're looking for.
One possibility could be that Naruto is somehow transported to the Avengers' world during the Age of Ultron. He might initially be seen as a threat due to his strange powers. But then he could prove his worth by using his ninja skills to fight Ultron's minions. His shadow clone jutsu, for example, could be very useful in confusing the robotic enemies.
Vision is definitely an interesting character. His unique origin as a combination of JARVIS, the Mind Stone, and Tony Stark's creation makes for a lot of potential exploration. Wanda Maximoff is also great. Her powers are complex and her backstory of loss and revenge can be delved into deeper. And of course, Ultron himself. His view of humanity and his extreme measures to 'save' the world can be examined from different angles.
Well, Reddit can be a great place too. There are various subreddits dedicated to Avengers fanfiction. You can post a request asking for good Avengers Age of Ultron fanfiction and the community will surely come up with some great suggestions. Also, Tumblr sometimes has some really interesting fan - made content including fanfiction related to the movie. You can search for relevant tags like #AvengersAgeofUltronFanfiction.