Maybe a plot where Robin and Harley get transported to an alternate dimension. In this new world, they have to rely on each other completely to find a way back home. Along the way, they face strange versions of their friends and foes, and their relationship evolves as they face these challenges together.
One interesting plot could be Harley Quinn trying to reform and Batman being skeptical at first. For example, she might start to help Batman take down some minor villains as a way to prove herself. Another plot could involve a team - up where they have to face a new, powerful enemy that neither could defeat alone. Maybe a villain who has the power to manipulate minds and they have to work together to resist the control.
Another plot might involve the Joker facing a new rival in the criminal underworld, and Harley Quinn has to choose between staying loyal to the Joker or making her own path. This could lead to a lot of drama and action, like Harley using her acrobatic skills and unique weapons to fight the new threat while dealing with her feelings for the Joker.
In a Harley Quinn x Superman fanfiction, there could be a plot where Harley Quinn is hired by someone to assassinate Superman. But as she gets to know him during her attempts, she realizes he's not what she thought. So instead, she decides to protect him. This change of heart could create a lot of drama and action as they face off against the forces that hired her in the first place. Also, it could explore how their different personalities - Harley's wildness and Superman's moral righteousness - interact and complement each other.
One interesting plot could be Luffy's newfound overpowered abilities helping him save Robin from extremely dangerous situations in a more spectacular way than in the original story. For example, he might be able to single - handedly take on an entire fleet of powerful enemies to rescue Robin.
Another plot could involve a time - travel scenario. Robin is sent back in time and meets a younger Maribelle. They form an unlikely friendship that over time blossoms into love. This causes some confusion as Robin has knowledge of Maribelle's future self, and they have to navigate the implications of their relationship on the timeline.
One interesting plot could be the Joker and Harley trying to raise their baby in a world that is constantly against them. Maybe they have to protect the baby from other villains who want to use it for their own gains.
A possible plot is that Harley gets caught in the middle of one of Peter's battles with a villain. Peter has to protect him while also trying to defeat the bad guy. This could lead to Harley learning about Peter's secret identity and then becoming his sidekick of sorts, using his own intelligence to assist Spider - Man in future battles.
One interesting plot could be Batman teaching Robin advanced combat techniques in a secret, underground training facility. Robin struggles at first but gradually masters the moves.
In some fanfictions, Robin might fight Batman due to a misunderstanding. For example, Robin could be misled by a villain's false information, thinking Batman has turned to the dark side. Another plot could be a power struggle within the Bat - family, where Robin wants more independence and challenges Batman's authority.
One interesting plot could be Robin and Starfire going on a secret mission together that tests their relationship. They might face a powerful enemy that forces them to rely on each other more than ever.