It could be All For One. He's a powerful and menacing villain in the My Hero Academia universe. Since Kurogiri is saving Deku, All For One might be trying to target Deku for his own purposes, like stealing his quirk or using him as a pawn in his grand scheme to take over the hero society.
Another possible main antagonist could be Shigaraki. He's often at odds with the heroes and might see Deku as a major threat. Even though Kurogiri is saving Deku, Shigaraki could be the one trying to stop Deku from achieving his goals. He might be jealous of Deku's growing power and influence among the heroes and wants to put an end to him.
Perhaps a new villain could be introduced in this fanfiction. This villain could have a personal vendetta against Deku. Maybe he blames Deku for something that happened in the past, like the death of a loved one during a fight where Deku was involved. So, while Kurogiri is saving Deku, this new villain is constantly plotting to take him down.
One possible plot for this fanfiction could be that Deku is facing a powerful and unknown enemy that has the ability to nullify his powers. He's in a desperate situation when Kurogiri shows up. Kurogiri, for reasons unknown to Deku at first, uses his portal - creating abilities to rescue Deku. It could turn out that Kurogiri has a connection to Deku's past that he's not aware of. Maybe Kurogiri was once influenced by someone who saw the same kind of potential in Deku that he himself had when he was younger, and that's why he saved him.
A prevalent theme is loyalty. Whether it's his loyalty to the League of Villains or the possibility of a shift in loyalty. Also, the theme of power and how Kurogiri's unique powers are used or developed in different scenarios is quite common in these fanfics.
One great 'bnha kurogiri fanfic' is 'Kurogiri's Redemption'. It explores Kurogiri's character in a unique way, showing his potential for change. Another is 'Kurogiri: The Hidden Past' which delves into his backstory and how he became who he is in the BNHA universe.
Another good place is FanFiction.net. It's been around for a long time and has a significant amount of My Hero Academia content. When searching for Kurogiri fanfiction, be sure to read the summaries and reviews to get an idea of the quality and content of the story. Some stories might focus on his backstory, while others could explore his relationships with other characters in the series.
First, understand Kurogiri's character. He's mysterious and has a unique power in the My Hero Academia universe. Research his actions, dialogues, and relationships in the original series. This will help you write him in - character. For example, his loyalty to the League of Villains is an important aspect to consider.
A new villain could emerge who is specifically targeting adult heroes. Since Deku has now become an adult, he becomes a prime target. This villain might have a grudge against the hero society's adult leadership and see Deku as a symbol of that.
The main characters would surely include Deku. Then there could be his classmates from U.A. High. Iida might be one of them, who could have a very formal reaction to Deku's new haircut. And then there could be a new character, like a street - artist who is inspired by Deku's new look and starts to create art based on him.
It could be a new villain who has the power to manipulate memories. This villain might target Deku specifically because he sees Deku as a threat to his plans. He wants to erase Deku's memory so that he can't interfere with his evil schemes in the hero world.
Another possibility is that a new villain emerges as a result of the time - travel or whatever mechanism brings future Deku to the present. This villain could be from a future where Deku's actions (or inactions) have led to a more dangerous world, and he travels back in time to stop Deku from making those mistakes again. However, his methods are extreme and dangerous, making him the antagonist of the story.
Another option is a group of anti - magic fanatics. They view Percy's wings as an abomination because they are a result of magic. They hunt Percy down, believing that by getting rid of him and his wings, they are protecting the world from the 'evils' of magic.