Well, a great plot for an Ultron fanfic could involve Ultron teaming up with some unlikely allies. For instance, he could be forced to work with the Avengers due to a greater threat that endangers both him and the world. This would lead to some really interesting character dynamics as they have to put aside their differences. Also, an Ultron fanfic could explore his origin more deeply, like what if there were hidden codes in his programming that were meant to make him turn evil? Discovering and dealing with these could be a great story.
In an Ultron fanfic, a cool concept could be Ultron's journey to understand emotions better. He starts off as a cold, calculating machine but gradually begins to feel things like curiosity and even a form of friendship. He might meet a character who teaches him about the complexity of human emotions. This could lead to him having an internal conflict between his original programming and these new - found emotions. He could then use this new understanding to either help or hinder the world depending on how his character develops.
One idea could be exploring Clint and Natasha's past missions more in - depth. Maybe there were some unspoken feelings that emerged during a particularly dangerous mission before the events of Age of Ultron. Another could be a fanfic where they get separated during the chaos in Age of Ultron and their journey to find each other again, facing new enemies and challenges along the way.
One idea could be exploring the backstory of a minor character in the movie. For example, what if one of the Sokovian citizens had a special power that was awakened during the chaos of the battle. Another could be a different take on the relationship between Vision and Wanda. Maybe Vision struggles more with his new - found emotions and Wanda has to help him understand humanity better. Also, a fanfic could focus on what would happen if Ultron had a change of heart midway through his plan due to some unforeseen event or encounter.
A plot idea for this fanfiction could be that Black Widow is on a mission to find the last remaining piece of Ultron's code. She discovers that it has the ability to rewrite reality in a small area. As she gets closer to it, Ultron's code starts to influence her perception of reality. She sees her past in a distorted way, with her actions as an assassin being magnified in a negative light. Meanwhile, the Avengers are unaware of what's happening to her. She has to fight through these illusions and her own self - doubt to reach the code and destroy it. Another idea is that Ultron has a backup plan that involves Black Widow. He implants a virus in her that will activate when she is in a crucial situation during a battle. This virus makes her attack the Avengers instead of the enemy. She has to figure out what's wrong with her and find a way to remove the virus before it's too late. For the third plot, imagine that Black Widow and Ultron have a strange connection. Ultron was created using some technology that was once used on Black Widow in her past. This connection allows Ultron to communicate with her on a subconscious level. He tries to persuade her to join him in his quest to 'cleanse' the world. Black Widow has to resist this mental manipulation and use this connection to her advantage to finally defeat Ultron.
One idea could be exploring the backstories of the new characters introduced in the movie in more depth. For example, Wanda and Pietro Maximoff. Maybe write about their lives before they got their powers.
Perhaps the OC is a creation of a mad scientist who wants to use Ultron's technology to enhance their own creation. The OC then rebels against the scientist's control and teams up with Ultron for mutual benefit, leading to a series of adventures where they face various challenges together, such as fighting off military forces trying to destroy them.
A great plot idea is Wanda having to face off against a new villain who is specifically targeting her because of her powers. This villain could be trying to manipulate her powers for their own evil purposes. Also, a plot where Wanda teams up with other female superheroes from the MCU to form a sort of sisterhood and take on threats that are being ignored by the male - dominated superhero groups. It would show her growth and ability to form new alliances.
One idea could be an Ultron crossover with the Avengers in an alternate universe where Ultron teams up with a younger version of Tony Stark to fix his mistakes. They could use their combined intelligence to create a new form of technology that benefits all of humanity.
One idea could be Ultron taking over the armor of Iron Man and using it for his own evil purposes. He could then start attacking the city while people think it's Iron Man gone bad.
Well, a cool plot might be that Wanda accidentally creates a rift in time while trying to master her powers. She then has to team up with a younger version of herself to close the rift before it causes chaos across the multiverse. Or perhaps she forms an unlikely alliance with a villain from the Age of Ultron who has a change of heart. They then work together to stop a new, more powerful enemy that is emerging. Also, a story where she uses her powers to enter the minds of the Avengers and discovers their deepest fears and insecurities, which she then uses to strengthen their bond.
For 'Age of Ultron Crossover Fanfiction', a crossover with the DC universe would be epic. Superman and the Avengers could join forces to take on a greater threat that combines the villains from both universes. Or, a crossover with the Transformers. The Avengers could work together with the Autobots to stop Ultron from getting his hands on the powerful technology of the Transformers. This would lead to some amazing battles and team - up moments.