It could be Orochimaru. He's always been interested in Sasuke and might see kidnapping Sakura as a way to manipulate Sasuke. He could use Sakura as a guinea pig for his experiments or as a bargaining chip to get Sasuke to come to him.
Well, one possibility could be that it's a dark fanfic where an enemy of Sakura kidnaps her to get to Sasuke. Maybe they want to use her as bait to lure Sasuke into a trap. Since Sasusaku is a popular pairing, the kidnapper might think that Sasuke would do anything to save Sakura.
There's a fanfic titled 'Kidnapped Sakura: Sasuke's Race Against Time'. It delves deep into the emotional turmoil Sasuke experiences when he discovers Sakura has been kidnapped. The story also showcases Sakura's strength even in the face of danger, as she tries to find ways to free herself while waiting for Sasuke to come to her rescue. The detailed descriptions of their emotions and the action - packed rescue scenes make it a great read.
If Sakura dies in a fanfic, it could have a huge impact on the whole Naruto -verse within that story. Her friends like Naruto would be deeply affected. Naruto might feel like he failed to protect her, just as he felt about other losses in his life. And for the medical ninja community in the fanfic, her death could be a big blow as she was a talented medic.
A common plot twist is that the kidnapper turns out to be someone Sakura least expected, like an old acquaintance with a hidden vendetta. Another could be that Sakura discovers she has a hidden power during her captivity that she uses to either escape or help Sasuke find her.
It might explore different aspects of Sasuke and Sakura's characters within the context of a fanfic. For example, it could show Sakura with different abilities or Sasuke having a different attitude towards her. Since it's a fanfic, the author has the freedom to deviate from the original storyline and create something new. It could also be about their relationship in a different time period or setting, like in a modern - day AU (alternate universe) or in a completely fictional world.
A fanfic could show Sakura sacrificing herself to save Sasuke. This would be a very emotional and heroic end for her. Sasuke would then have to live with the guilt and the memory of her selfless act.
I'm not sure specifically about a 'Sasusaku kidnapped lemon' fanfiction. There are so many fanfictions out there that it's hard to say for certain. You could try searching on popular fanfiction websites like FanFiction.net or Archive of Our Own.
I'm not entirely sure as I haven't read this specific fanfic. But generally, in SasuSaku (Sasuke and Sakura from Naruto) fanfics, it could be about some new adventure that breaks Sakura's seal and how that impacts her relationship with Sasuke and the overall story world.