A really interesting plot could be that Natasha and Steve find a lead on a powerful weapon that Ultron is after. They race against time to secure it before Ultron does. Along the way, they encounter various challenges and enemies. Their different fighting styles and personalities come into play as they strategize. Steve's more straightforward and honorable approach might clash with Natasha's more devious spy tactics at first, but they learn to combine their strengths. As they get closer to the weapon, they also realize that there is a deeper conspiracy behind Ultron's actions that involves some of their old enemies.
One plot idea could be Natasha and Steve having to go undercover together during a mission related to Ultron's early schemes. They could pretend to be a couple, which would lead to some awkward yet heartwarming moments as they navigate their feelings while also trying to stop Ultron. Another idea is that Natasha discovers a hidden past of Steve that is somehow connected to the creation of Ultron and they have to deal with the revelations together.