One possible plot could be that the OC wakes up as Natsu in a world where Fairy Tail is facing a new, extremely powerful dark guild. Since the OC has their own knowledge and personality, they might use Natsu's powers in unique ways. For example, they could combine Natsu's fire - based attacks with strategic thinking that is different from the original Natsu. This could lead to new and exciting battles and alliances within the Fairy Tail guild as they try to figure out why Natsu seems a bit 'off' but is also more effective in some ways.
Maybe the OC wakes up as Natsu and discovers that Natsu has some hidden powers or a secret past that was previously unknown. The OC, in Natsu's body, has to navigate through this new information while also dealing with the everyday adventures and challenges in the Fairy Tail world. They might have to face some of Natsu's old enemies who sense a change in him, and the OC has to bluff their way through using Natsu's bravado and skills until they can fully understand and control this new situation.
A plot could be that the OC wakes up as Natsu right before a big tournament. Natsu was expected to participate, but now the OC has to take his place. The OC, not being fully used to Natsu's body and abilities at first, struggles during the early matches. However, as they go along, they start to adapt and use Natsu's natural fighting instincts along with their own creativity. They might also form new relationships with other guild members during the tournament, and in the end, they not only do well in the tournament but also bring a new sense of unity to the Fairy Tail guild.
One possible plot could be that the OC wakes up in the world of Hayley Marshall and has to quickly adapt to the supernatural elements. Maybe they first struggle to understand their new powers, if they have any. They could end up becoming friends with Hayley and helping her in her battles against her enemies. For example, they might assist in protecting the werewolf pack from a new threat in the area.
Suppose the OC is more introverted. In Natsu's body, they might have a hard time dealing with Natsu's boisterous friends at first. This could create some comical and also some tense moments in the story. But as time goes on, the OC might start to open up a bit more because of the influence of Natsu's friends, and in turn, they could also influence the guild members in new ways, like introducing quieter, more thought - out strategies during missions.
In a 'natsu married oc fanfiction', perhaps the OC is a healer. Natsu gets seriously injured during a battle and the OC nurses him back to health. Natsu is so grateful and impressed by her kindness and skill that he starts to develop feelings for her. They get married and the story could focus on how Natsu has to adjust to having a partner who is more calm and collected compared to his fiery self. He might learn to be more patient and she might learn to be more adventurous from him.
Perhaps he wakes up to find that the laboratory is in the middle of a big experiment gone wrong. There are strange creatures or phenomena around him, and he has to figure out how to escape and also stop whatever bad thing is about to happen. He might use his Pokémon knowledge and skills to get through this tough situation.
In the fanfiction, when Ash wakes up in the room, he might discover that he has new, unexpected powers. The room could be a sort of testing ground for him. He'll have to learn to control these powers while also dealing with the mysteries of the room, like hidden doors and cryptic messages on the walls.
Happy could be an interesting character. He could be like a sidekick and a source of comic relief during their relationship journey. For example, he might accidentally interrupt their romantic moments or give Natsu really bad advice on how to court the OC at the beginning. Erza could also be important. She could be the one who gives the OC advice on how to handle Natsu's wild nature, as she has known him for a long time.
One possibility is that Naruto would initially be confused and struggle to control his new form. He might accidentally cause destruction in the village as he tries to figure out his new powers. But with the help of his friends, like Sasuke and Sakura, he could start to learn to harness the tailed beast's abilities for good, perhaps even developing new jutsu.
One possible plot could be that Tyrion, now young again, uses his knowledge of the future to his advantage. He might try to change certain events that led to his hardships in his previous life. For example, he could work on improving his relationship with his family from the start, especially with his father. Maybe he'd use his wits in a different way, to gain more respect and power in Westeros without having to go through the same level of humiliation and suffering.
One interesting plot development could be Bella struggling to control her new vampire powers. For example, she might accidentally use her super speed and crash into things at first. Another could be her dealing with the changed relationships. Since she's now a vampire, her connection with her human friends and family becomes more complicated as she has to hide her true nature from them. Also, there could be a plot where she discovers new vampire abilities that are unique to her, which leads to her having a special role within the vampire community.
In this fanfiction, when Sakura wakes up in the Konoha forest void, it could be that she is the only one who can sense the true nature of the void. Perhaps it is not just an empty space but a gateway to another dimension or a prison for some powerful entity. She may start a journey to either close the gateway or free the entity if it is proven to be good. This would involve her using her medical ninjutsu knowledge in combination with her combat skills to deal with any threats that come her way.