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.
His father, Tywin Lannister, could still be the main antagonist. Even though Tyrion is young again, Tywin's views and treatment of Tyrion might not change. He could still see Tyrion as a disappointment and an embarrassment to the Lannister name, and thus continue to oppose Tyrion's every move.
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.
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 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.
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.
One possible plot could be that they are forced into a political marriage and Sansa becomes pregnant unexpectedly. This would create a lot of drama as they deal with their complex relationship and the reactions of those around them.
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.
One possibility is that he wakes up and discovers new allies. The story could then revolve around him building relationships with these new characters while still holding onto his old values. He may also have to deal with the fact that the world has moved on without him and find his new place in it.
Well, it could be about a magical spell. Maybe he offended a witch and she turned him into a girl as a punishment. He then has to go through his day trying to figure out how to reverse it while dealing with all the new experiences of being female, like wearing different clothes and having different social interactions.