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.
One idea could be creating a robot OC that has the ability to transform into different forms inspired by the various Grimm in RWBY. For example, it could shift into a Beowolf - like form for combat situations. Another idea is a robot OC designed by a villainous character in the RWBY universe but ends up having a change of heart and joins the heroes. You could also have a robot OC that has a unique Dust - based power system, which is different from the normal ways Dust is used in RWBY.
Another interesting concept is a robot OC that was created by a villain but has a change of heart. This robot could struggle with its programming and new - found sense of morality. It might have to hide its true identity from both heroes and villains while trying to do good in the world of BNHA.
The OC could be a therapist who is assigned to treat Elliot. At first, Elliot is very resistant to the idea of therapy. However, the OC is persistent and gradually breaks through his walls. As they interact more, the OC starts to understand Elliot's complex mind and the world of hacking he is involved in. This leads to a romantic connection as the OC becomes a part of Elliot's world and helps him in his fight against Evil Corp.
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.
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.
A great Ultron OC fanfiction might involve an OC who is a time - traveler. They go back in time to stop Ultron from being created in the first place. Along the way, they interact with the people involved in Ultron's creation, like Tony Stark, and try to change the course of events without disrupting the timeline too much.
Vision could be a great character to pair with an Ultron Robot OC. Since Vision was created from Ultron's original plan, there could be a lot of interesting interactions. The OC could either befriend Vision or be in conflict with him, depending on the nature of the OC.
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.
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.