One idea could be that Himiko Toga kidnaps Midoria because she wants to understand his power better. She might be curious about how his unique abilities work and thinks that by keeping him close, she can study him. Maybe she takes him to an abandoned building where she has set up a sort of makeshift lab to run tests on him, all while Midoria is trying to figure out a way to escape and get back to his friends.
Well, one possibility is that Toga kidnaps Deku to get closer to the heroes. Maybe she thinks she can use Deku as a bargaining chip to make the heroes do something for her. For example, she might demand the release of some of her villain friends from prison.
His friends could come to the rescue. Bakugo, for instance, might use his explosive power to break into Toga's hideout. He's always been competitive with Deku, but he also cares about him deeply.
Midoriya would initially be in shock. But then, his heroic instincts would kick in. He might try to analyze Toga's behavior and find a way to use it against her. For instance, if he notices that she gets distracted easily, he could create a diversion. He would also be thinking about the safety of others. He might worry that Toga has some bigger plan that involves harming his classmates or the city. So he would be constantly looking for a way to not only escape but also stop Toga's potential evil plans. And throughout all this, he would be drawing on his training at U.A. to keep his composure and think rationally.
In a Toga kidnaps Midoriya fanfiction, the plot could go in various directions. Maybe Toga kidnaps Midoriya to trade him for something she wants from the heroes, like the release of a fellow villain. Midoriya, on the other hand, could try to escape by using his wits and his knowledge of Toga's weaknesses. For example, if he figures out that she has a soft spot for a certain type of person or situation, he could use that to his advantage. Also, the story could involve the other students at U.A. trying to rescue Midoriya. They might come up with a clever plan that involves using their individual quirks in a unique way to outsmart Toga and get Midoriya back.
Fanfiction often takes liberties with the relationship between Izuku and Toga. It could also involve them teaming up against a common enemy. This would require them to put aside their differences and work together. Writers can use this to create exciting action - packed scenes and explore how they would communicate and cooperate in such a situation.
Toga is still alive in the manga. The creators haven't decided to end her character's journey with death. Usually, such major plot points are well-signaled and haven't happened for Toga so far.
There is also 'Toga Himiko: The Redemption'. This fanfic delves deep into the idea of whether Toga can be redeemed. It has great character development and a unique take on her relationships with other characters in the series.