Bakugo might initially be in denial. He's always had a complex relationship with Izuku, and seeing him in such a state could shatter his world view. He might become more withdrawn or, on the contrary, be spurred into a violent rage against whoever was responsible.
One common theme is the struggle for acceptance. Since she's a female in a world where the main character was originally male, she often has to prove herself more. Another theme is friendship. Female Izuku's relationships with her friends, like Bakugo or Todoroki, are often explored in new ways.
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.
A villain like Shigaraki would likely be interested in the power levels in the MCU fanfiction. He's always looking for ways to increase his own power, and seeing the various powerful artifacts and abilities in the MCU might give him some ideas. He could think about how he could use similar things in his own plans to take over the MHA world.
One popular MHA female Izuku fanfiction is 'She - Izuku Midoriya'. It offers a unique take on Izuku being female and explores how her character would interact differently in the MHA universe. Another great one is 'Female Izuku: A New Heroine'. It delves into the challenges she might face as a female in a male - dominated hero world and how she overcomes them.
One possible consequence could be the extreme shock and grief among his friends and allies. They would likely be devastated and it could lead to a downward spiral in their morale. For example, his classmates like Ochaco might blame themselves or become extremely angry at the situation.
Aizawa may also interact with Izuku's family during family events or gatherings in the fanfic. He could be a guest and during that time, he could share his insights on Izuku's growth and development. This would not only strengthen his relationship with Izuku but also with his family as a whole.
Some in 1A might be in awe. For example, if Izuku has a particularly powerful or unique quirk in fanfiction, characters like Mineta might be jealous. He often shows envy towards those with strong abilities.