Some of the side characters like Shikimori's friends might also gain popularity in the fanfiction. If the fanfiction gives them more depth and interesting storylines, readers could really get attached to them. For example, a friend who has a hidden talent or a mysterious past.
Another plot could be a time - travel scenario where Shikimori goes back in time and has to deal with her past self and the challenges that come with it. This could add a whole new layer of complexity to her character.
The unique characters make it popular. Shikimori herself is a very interesting character with her cool and cute sides. Her relationship with her boyfriend is also quite charming, full of sweet and funny moments.
The story is about a relationship. Shikimori's boyfriend often gets into some small troubles, and Shikimori is there to save the day with her unexpected coolness. It also shows their daily life at school and with friends, filled with warm and humorous moments.
All Might would be furious. He would see it as his failure to protect Izuku. He would probably mobilize all the resources he has, both in his hero form and his connections in the hero world, to rescue Izuku. And he would be blaming himself all the while for not being there when Izuku was kidnapped.
Some characters might be immediately terrified. They could avoid Izuku or even try to drive him away. For example, classmates who don't understand his quirk might spread rumors about him.
Izuku would probably be terrified at first. After all, he's been kidnapped by villains. But then, his hero - like determination would kick in. He might start looking for ways to escape or at least resist their plans.