One possibility is that Harry grows up in the Muggle world completely unaware of his wizarding heritage. He might lead a normal life, going to a regular school, making Muggle friends, and having no idea about the magical world that could have been his. Maybe he has a natural aptitude for something like martial arts or a particular academic subject that in the wizarding world would be replaced by magic.
One possible plot could be that Harry Potter discovers a different magical school. Maybe he stumbles upon a hidden school in the Muggle world that has been secretly teaching magic. He could form new friendships and face different types of magical challenges there. It would be a completely new magical experience for him, away from the familiar Hogwarts environment.
Perhaps she discovers a different magical school or a secret magical community outside of the Hogwarts system. In this fanfiction, she could be mentored by an eccentric and powerful wizard who has his own way of teaching magic, far removed from the traditional Hogwarts curriculum.
Well, Deku would probably be super excited to learn new kinds of magic at Hogwarts. He might struggle a bit at first with the different system compared to his quirk training, but his determination would make him study hard. Bakugo would likely be a bit of a troublemaker, challenging the wizarding rules and maybe getting into some minor duels with other students. Ochaco would be making friends left and right and might use her zero - gravity quirk in some creative ways during flying lessons on brooms.
One possibility could be that Snape grows up in a different magical environment, perhaps in a small, hidden magical community that has its own unique way of teaching magic. He might develop his skills in potion - making and the Dark Arts in a more self - taught or community - guided way. Without the influence of Hogwarts, his relationships would be very different. He may not have the same intense rivalries or unrequited love for Lily Potter.
Well, he'd probably be a bit of an oddball at first. His skills with a sword and his water - based powers would be really different from the magic at Hogwarts. He might get into trouble for using his powers instead of spells. But he could also be really good at things like Defense Against the Dark Arts because he's used to fighting monsters.
The Dumbledore character could still be important. He might be secretly watching over Harry, guiding him from a distance. Another character could be a new wizard or witch that Harry meets in the Muggle world who has a special mission related to Harry. For example, a young witch who has been exiled from the wizarding world but has the knowledge to help Harry understand his powers. And of course, Voldemort would still be a main antagonist as his threat looms over the entire wizarding world whether Harry is at Hogwarts or not.
In 'deku meets future deku fanfiction', perhaps the future Deku warns the present one about upcoming threats. He could tell him about villains he'll face in the future and how to defeat them. This would give the present Deku an edge in his battles and also in his growth as a hero.
One possible plot could be that Villain Deku is on a mission to undermine the hero society and crosses paths with Hero Deku. They engage in a fierce battle where Villain Deku taunts Hero Deku about his ideals. In the end, Hero Deku manages to get through to a small part of Villain Deku's conscience, leaving him with some doubts about his villainous path.
Another plot might involve the older Deku being from an alternate timeline. He shows the younger Deku different ways he could have developed his skills and relationships, which makes the younger Deku reevaluate his own path as a hero.