He might meet the future versions of Naruto and Sakura. Their relationship with him could be very different in the future, perhaps they are now leaders of the village and Sasuke has to adjust to their new status.
Sasuke could also be motivated by power. He might believe that Sakura has some sort of knowledge or power that he needs. By kidnapping her, he can force her to give it to him or help him obtain it. This could be related to her medical skills or some hidden power within her that Sasuke has discovered.
Since 'hard lemon' fanfiction is inappropriate, we can talk about their general characteristics in normal fanfiction. Sasuke is often depicted as a brooding and powerful character, while Sakura is shown as strong - willed and loving. In non - inappropriate fanfiction, their relationship might be shown as evolving from initial acquaintance to deep love, with Sasuke's cold exterior gradually melting due to Sakura's unwavering affection.
Sasuke would likely be aloof and guarded at first. He might see Batman as just another 'vigilante' like some of the shinobi in his world. Batman, on the other hand, would be cautious and try to figure out Sasuke's motives. Eventually, they could start sharing their combat techniques. Sasuke might show Batman how to use chakra in combat, and Batman could teach Sasuke about the importance of having a strong moral code.
I think young Sasuke would have a very intense reaction. He was so consumed by his hatred for Itachi at that young age. A Sasuke vs Itachi fanfiction would be like a trigger for him. He would be eager to see how the fictional fight is portrayed. If Sasuke in the fanfiction is shown as strong, it might fuel his own ego and make him strive even harder to become powerful. However, if the fanfiction shows Itachi as superior, it could enrage him. He might feel that it undermines his own goal of defeating Itachi and he might even want to find the writer to correct their 'wrong' view.
One common theme is Sasuke's struggle to admit his feelings. When he's jealous, it shows that he actually cares more about Sakura than he lets on. Another theme could be misunderstandings. He might misinterpret Sakura's interactions with others as something more than friendship.
Sasuke would likely be initially shocked. He's a man who has been through a lot and is used to a life of solitude and danger. But then, a sense of protectiveness would kick in. He'd become more vigilant about the safety of Sakura and their unborn child. He might also start to think about how to be a better father than his own was, in some ways, trying to break the cycle of his family's past.
In these fanfics, one characteristic is the growth of their relationship. At first, Sasuke may be aloof, but gradually he opens up to Sakura. Sakura, on the other hand, is always steadfast in her love. Another is the mutual support. They often face difficulties together, be it in battles or in rebuilding their world, and they rely on each other's strength. Also, there's the element of deep understanding. They come to understand each other's past traumas and insecurities, which brings them closer.