Another plot could revolve around Yo Shindo's past. Perhaps he has a family secret that is related to his quirk. A fanfiction could explore how he uncovers this secret and how it changes his perception of himself and his place in the hero world. It could also involve his family members having special abilities that were kept hidden for some reason.
One plot idea could be Yo Shindo getting involved in a secret mission. He discovers some hidden conspiracy within the hero society and has to use his wits and quirk to uncover the truth. Maybe there are villains operating from within the shadows, and Yo Shindo is the only one who can sense their presence due to his vibration - based quirk.
A plot idea could be Yo Shindo participating in an international hero exchange program. He travels to a different country and has to adapt to a new culture while also facing new types of villains. This would be an interesting way to see how his quirk fares against different threats and how he interacts with heroes from other parts of the world.
Yo Shindo in My Hero Academia has a lot of potential for interesting fanfiction. His unique quirk makes for great storylines. For example, a fanfiction could focus on his training to better control his vibrations. He could face challenges that force him to push his quirk to new limits, which would lead to his character growth both in terms of power and personality.
How about a fanfiction where the villains and heroes have to team up? There could be a greater threat, like an alien invasion or a powerful quirk - user who is out to destroy the entire world. The heroes and villains realize that they can't defeat this threat alone, so they put aside their differences. This would lead to some really interesting character interactions, like Bakugo having to work with Shigaraki and both of them learning to respect each other's skills despite their opposing natures.
An interesting plot could involve an anti - hero who has a hidden agenda that is not entirely evil but also not in line with the traditional hero goals. Maybe they are trying to expose a corrupt part of the hero society from within. They could form alliances with some villains who also have a vendetta against the corrupt elements. This would create a complex web of relationships and moral dilemmas.
One plot idea could be a villain who is secretly a double agent, working for the heroes all along. They infiltrate the villain group and gather information. Another idea is a hero who loses their quirk and has to rely on their wits and combat skills alone, like Batman in the DC universe. And a villain - hero team - up to face a greater threat that neither can handle alone.
Maybe a plot where a group of Warhammer heroes are sent to the Hero Academia world to stop a chaos invasion that is starting to seep into that realm. They have to work with the local heroes, sharing their knowledge of dark magic and ancient weapons to combat the new threat. This would involve a lot of cultural clashes and learning experiences between the two groups.
A different idea is that some of the students from U.A. High School go on a field trip to Bikini Bottom. There, they learn about the different 'quirks' the underwater creatures have, like Sandy's ability to live underwater as a squirrel. And at the same time, they have to help SpongeBob and his friends with a problem, like stopping a giant sea monster that the normal underwater heroes can't handle. This would involve a lot of teamwork between the two groups.
An interesting plot could involve an OC who is initially thought to be quirkless but discovers a latent quirk later on. This OC could then struggle to catch up with the others in terms of training and development. Or, an OC with a shapeshifting quirk could be used in a plot where they have to infiltrate a villain's lair by taking on the form of one of the villains' minions. They would have to be careful not to be discovered and also gather intelligence along the way.
One idea could be a crossover where the students from Little Witch Academia visit UA High. They could participate in a joint training exercise. The witches could use their magic in creative ways during combat scenarios, while the heroes-in-training show off their quirks. It would be a fun mix of different powers and personalities.
One plot idea could be that the Saiyan OC is mistaken for a new type of villain at first. As they try to prove they're not a threat, they start to make friends and learn about heroism. Another idea is that they discover a connection between Saiyan powers and some of the hidden secrets in the MHA world. Maybe there's an ancient power source that only a Saiyan can unlock.
One cool quirk idea could be 'Shadow Mimicry'. The user can blend into any shadow and move freely within the shadow realm, also being able to mimic the actions of those whose shadows they are in. It gives a lot of stealth and surprise attack possibilities in a fanfiction.