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.
Several characters have met their demise in the MHA manga, but it's a bit complex to list them all precisely. Some of the notable ones include [mention specific characters if known].
I think it's hard to say for sure. There are a few characters who could potentially be the traitor, and it's all part of the mystery and suspense of the story.
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.