A plot idea could be that a portal opens between the My Hero Academia world and the DC universe where the Speed Force exists. A hero from MHA gets sucked into the DC world and has to learn to control the Speed Force to get back. Along the way, they meet DC characters and learn new ways to use their speed - based powers.
Well, in a My Hero Academia Speed Force fanfiction, a character could be born with a quirk that is actually a latent form of the Speed Force. They might not realize it at first, but as they grow and face challenges, their connection to the Speed Force starts to emerge. This could involve things like accidentally phasing through objects because of extreme speed, or experiencing time dilation during intense battles.
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.
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 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 idea could be a 'what if' scenario where the villains and heroes have to team up against an even greater threat from another dimension. It would be fun to see how characters like Bakugo and Dabi interact in this forced alliance.
One idea could be that during Halloween, the villains use the spooky season as a cover to carry out a major heist. The heroes have to figure out their plan while also dealing with all the Halloween chaos in the city. For example, there could be trick - or - treaters getting in the way and the villains using Halloween decorations to hide their traps.
One idea could be reincarnating as a minor villain and trying to reform. For example, using knowledge of the future to avoid getting caught up in big evil plans. Another could be reincarnating as a quirkless student but finding a unique way to be a hero, like developing amazing combat skills without a quirk. Maybe reincarnating as a side character and becoming the mentor to the main heroes, guiding them with knowledge from the original story.
One idea could be that a kaiju suddenly appears in the My Hero Academia world and the heroes have to figure out a new way to combat it since their usual powers might not be as effective. For example, they might have to work together in a more strategic way than ever before, combining different quirks to create a new type of attack.