A really interesting plotline could be a love triangle situation. Bucky remembers his past feelings for Natasha, but Natasha has developed a deep bond with Steve in the meantime. This could create a lot of tension and drama, with the three of them having to navigate their relationships while also dealing with the threats and challenges that come their way in the Marvel universe.
Perhaps a plotline where Bucky's memories of Natasha lead to him uncovering a hidden secret about her past. This could involve some sort of mystery related to her training in the Red Room or her family. Steve might be involved in helping Bucky understand these new revelations and how they affect Natasha's present situation.
One possible plotline could be Bucky's internal struggle as he remembers Natasha. Maybe he has memories of a mission they were on together before his mind was messed with. He could be conflicted about his feelings for her now that he's back to his senses, especially with Steve also having a connection to Natasha.
Fanfiction.net is also a good place to look. It has a large collection of fanfictions. Navigate to the Marvel section and then start looking for stories with the specific tags or keywords related to Steve, Natasha, and Bucky. However, the search function there might not be as advanced as AO3, so you might need to do a bit more digging.
One great plotline could be Natasha and Steve teaming up in a covert mission during the events of Infinity War that wasn't shown in the movie. They might discover a hidden weakness of Thanos and try to exploit it. Another could be a story focused on their relationship development in the midst of the chaos of the war, like how they support each other through the tough decisions they have to make.
I really like 'natasha x bucky fanfiction' titled 'The Silent Bond'. It has a lot of action scenes combined with their budding relationship. The author does a great job of showing how their skills complement each other during missions and how that leads to a deeper connection off - mission.
One interesting storyline could be about their origin stories. How Steve became Captain America and Natasha's past as a spy. The fanfiction could explore how these different backgrounds affect their relationship. For instance, Steve's idealism might clash with Natasha's more pragmatic view at first, but then they learn from each other.
One great Natasha and Steve married fanfiction is 'Our New Beginning'. It beautifully portrays their life after marriage, showing how they adjust to living together and handling the challenges of being a super - couple. Another is 'Married Life of Nat and Steve' which focuses on their more domestic moments, like cooking together and sharing stories from their past missions.
Another great one is 'Unlikely Allies'. It delves into how Steve and Natasha, who are very different in many ways, find common ground and eventually fall in love. It shows their journey from just being teammates to something more, with detailed descriptions of their interactions and internal thoughts.
In the fanfiction, Steve and Natasha could be students at Avengers University. Steve is a transfer student from a military - like academy and Natasha has been at the university for a while. They meet in a combat training class. Steve's raw strength and Natasha's finesse catch each other's attention. They start training together outside of class to improve their skills. During this time, they start to develop feelings for each other. However, they have to hide their relationship from the strict university rules about student - student relationships.
It could start slowly. Maybe they first notice each other during a lecture at Avengers University. Steve is intrigued by Natasha's quiet confidence, and Natasha is curious about Steve's old - fashioned values. They gradually become friends as they study together in the library. As time passes, they find themselves thinking about each other more and more, and eventually, their friendship turns into a romantic relationship.
One great fanfiction could be 'Bucky and Natasha: Allies in the Shadows'. It focuses on their relationship during the events of Civil War, showing how they team up secretly to uncover hidden agendas. Another is 'Natasha's Bond with Bucky', which delves deep into their emotional connection, perhaps born out of their shared experiences of having a complex past.
A popular one could be 'Bucky and Natasha: Infinity War Redemption'. In this fanfiction, they both see Infinity War as a chance to redeem themselves for their past actions. Bucky wants to make up for his time as the Winter Soldier, and Natasha feels she needs to prove herself after all her morally grey deeds. Together, they embark on a journey that changes the course of the war in unexpected ways.