One idea could be exploring how Ultron's advanced technology interfaces with Sigma's unique abilities. Maybe Ultron tries to assimilate Sigma's powers for his own evil plans.
First, do research on both Ultron and Sigma. Know their backstories, abilities, and personalities well. This will help you create a believable interaction between them.
Captain America might be paired with Ultron as well. Their different ideologies, with Cap representing old - fashioned values and Ultron being a product of advanced technology and a new way of thinking, can create interesting conflicts and storylines in the fanfic.
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.
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.
First, know the characters well. For Ultron, understand his motives, his intelligence, and his capabilities. When crossing him over, find a logical connection. For example, if crossing with Guardians of the Galaxy, Ultron might be interested in the powerful artifacts they encounter. Then, create a good plot that challenges the characters and makes them grow. Don't forget to add some action and dialogue to keep the story engaging.
For an 'Age of Ultron Crossover Fanfiction', if crossing with the Power Rangers, the Rangers could bring their color - coordinated teamwork and powerful zords. Their positive attitude and ability to work as a team could inspire the Avengers. Another character could be Goku from Dragon Ball Z. His super strength and ability to go Super Saiyan could be a huge help in the fight against Ultron. He could also have some interesting battles with the stronger Avengers members.
One interesting concept could be the interaction between Ultron and characters from different universes. For example, how Ultron would react to superheroes with completely different powers and moral compasses.
One possible scenario could be that Quicksilver uses his super - speed to outmaneuver Ultron's attacks in the crossover. Maybe he teams up with both the X - Men and the Avengers, bringing his unique skills to the fight. For example, he could quickly disrupt Ultron's robotic minions or even steal important components from Ultron himself while the other heroes engage in direct combat.
Spider - Man could use his agility. He could quickly dodge Ultron's attacks and find a weak spot in Ultron's armor. Since he's so fast and nimble, he could get in close and plant some sort of disabling device he made with his scientific knowledge.