His closest friends like Ochaco Uraraka and Tenya Iida would most likely side with him. They have always been there for Midoriya and share his values of justice and doing what's right. Also, those heroes who have seen Midoriya's growth and believe in his pure intentions would probably join his side.
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.
Momo Yaoyorozu is also often paired with Izuku in married fanfiction. Given her intelligence and Izuku's growth as a hero, they can be a powerful couple. They might work together on strategic plans to defeat villains or improve hero education in the fanfictions. It's really fascinating to see how their different abilities complement each other in these stories.
A vengeful villain whose plans were once thwarted by a hero could be the kidnapper. He might think that by kidnapping Midoriya, he can draw out the hero and get his revenge. Midoriya is just an innocent pawn in his evil scheme.
One common plot device is the discovery plot. Midoriya might find out in a very dramatic way that Harry Potter is his father. For example, he could find a hidden letter or a magical object that reveals the truth. Another device is the mentor - like role of Harry Potter. Since he's so experienced in magic, he could teach Midoriya some special skills related to his quirk that has a magical origin.
Obviously, Midoriya is the main character. There could also be his friends from his original story like Bakugo. Bakugo might be jealous of Midoriya's cork at first and then later come to understand its significance. Another possible character could be All Might. He could be a mentor figure to Midoriya, guiding him on how to use whatever advantage the cork gives him.
Definitely not. Midoriya's character is very much alive in the manga. The creators have no intention of killing him off as he's a key figure in driving the narrative forward and inspiring other characters.
Well, in fanfiction, anything can be made possible through creative reinterpretation. Maybe there could be some sort of magical cross - over event. For example, a powerful wizard might accidentally create a portal between the Harry Potter universe and the My Hero Academia universe where Harry Potter and Midoriya's mother meet. However, this completely goes against the established canons of both series, but that's the beauty of fanfiction - it breaks the rules to create new and interesting stories.