It could be about Tobi having some serious motives for tying up Sakura. Maybe he's trying to extract information from her, thinking she has knowledge that is crucial for his plans.
In the 'serious tobi ties up sakura fanfiction', it might be that Tobi believes Sakura is a threat to his grand scheme. He ties her up to keep her from interfering. It could also be a misjudgment on his part. He may think she is more powerful or involved in certain events than she actually is. This situation could lead to Sakura using her wits and medical ninjutsu skills to try and escape, or it could be the start of a complex relationship where Tobi realizes she's not what he initially thought and they have to work together in some unexpected way.
The main characters are clearly Tobi and Sakura. Tobi is the one doing the tying up, which implies he has some sort of agency in the story. Sakura, on the other hand, is the victim of this action, and she will likely be the one trying to either resist or escape from this situation.
It could be that in this fanfiction, Black Zetsu ties up Sakura as a way to stop her from interfering with his greater plans. Since Sakura is a strong and capable ninja, immobilizing her in this way gives Black Zetsu an advantage. He might be trying to use her as a bait to lure in other characters, like Naruto or Sasuke, into a trap.
One possible reason could be a misunderstanding. Maybe Sasuke has been misled to think Sakura is a threat to him or his goals at that moment. In the Naruto universe, Sasuke has often been on a solitary path filled with suspicion. He might tie her up to keep her from interfering with something he's about to do. Another aspect could be related to some kind of test. He may want to see how Sakura will react in such a difficult situation to gauge her growth as a ninja. It could also be a part of a plan to protect her from something more dangerous, by restraining her movement in a way that he believes is the best for her safety, using the snakes as a means to immobilize her as he is known for his snake - like jutsu.
In this fanfiction, it could be that Sasuke has lost his mind due to some traumatic event. He ties Sakura with snakes because he sees her as a symbol of the world he wants to reject. The snakes could represent the chaos and danger that he now associates with her. Meanwhile, Sakura might be in shock at first, but then she starts to analyze the situation. She realizes that Sasuke is not in his right mind and tries to use her medical knowledge and ninja skills to not only free herself but also to heal Sasuke's broken psyche.
One possible plot could be that Tobi, Sakura, and the female reader all end up on a mission together. Tobi's unpredictable nature might create some comical situations at first. Sakura, with her medical and combat skills, would be a reliable ally. And the female reader could have some unique abilities that help them overcome the challenges in the mission, like perhaps the ability to communicate with animals which gives them an edge in a forest - type mission.
One possibility could be that Percy ties up Annabeth as part of a prank gone wrong. Maybe he was trying to play a joke on her for some past mischief she caused him, but then things spiral out of control and he has to figure out how to untie her before they get into real trouble.
One possible plot reason could be a misunderstanding. Maybe Draco thinks Hermione has some information that she's not sharing and he ties her up to force her to talk. In some fanfictions, it could be part of a game or a dare within the magical world they inhabit. Another reason might be that Draco is being influenced by some dark magic or a curse that makes him act out of character and tie Hermione up.
It could be that Gaston ties up Kairi and Belle out of jealousy. He's jealous of the attention they receive from others. In the story, while they are tied up, they could recall their own past experiences of facing difficulties. Kairi might think about her adventures with Sora and how she overcame challenges. Belle could think of the time she tamed the Beast. This reminiscing could give them the strength and motivation to break free. They might start by trying to call for help in a way that only their friends would understand, like a special whistle or a call that has a hidden meaning. Then, when their friends arrive, they could all join forces to deal with Gaston.
Well, in 'itachi ties up sakura fanfiction', a recurring theme might be the exploration of the characters' hidden sides. Since this is a non - canon scenario, writers can delve into aspects of Itachi and Sakura that were not fully explored in the original series. For example, Itachi's more controlling or possessive side, and Sakura's resilience in a difficult situation. There could also be themes related to trust and betrayal, as Sakura may initially feel betrayed by Itachi's actions but then things could develop in unexpected ways.
I don't think it's a common fanfiction plot. While there are all kinds of fanfiction out there, the idea of Pegasus's goons tying up Sakura is rather specific and not something you'd typically see in mainstream fanfiction. However, in certain sub - genres that deal with more action - oriented or darker themes, it might pop up, but it's not a widespread or common type of plot overall.