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.
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.
One popular toga fanfic might be those that explore her backstory in more depth. For example, a fanfic could focus on her life before joining the League of Villains, her relationships with other characters in that context, and how she developed her unique style and skills.
Toga could be kidnapping Midoria out of envy. Midoria has so many friends and supporters, and his growth as a hero is remarkable. Toga, who may feel alone and misunderstood in her own villainous world, wants to take away what makes Midoria so special. So, she kidnaps him in a fit of jealousy, but Midoria's positive attitude might start to change her perspective during the captivity.
A third common theme is the power dynamic between them. Dabi has a certain level of power and mystery, and Toga has her unique abilities. Fanfic may play with how these powers interact in different situations, such as in battles together or in protecting each other. This can add an interesting layer to their relationship in the stories.
A toga himiko fanfiction might also focus on her past. Maybe it delves into her childhood, what led her to become the person she is. It could show her experiences growing up that shaped her into the complex character we see in the original work, and how those early events still influence her actions in the fanfiction world.
One popular 'deku x toga fanfic' is 'A Different Kind of Bond'. It explores an interesting dynamic between the two characters where they start to understand each other beyond their initial differences. It has great character development and a unique plot.