A cool plot could be that Godzilla is actually an ancient being that was sealed away long ago in the My Hero Academia world. Due to some villainous activity, the seal is broken. Now, the heroes not only have to deal with the immediate threat of Godzilla but also figure out how to reseal it. There could be a sub - plot where a young hero discovers an old prophecy about Godzilla and has to convince the others that it's real and relevant to their current situation.
One idea could be that the Pokemon world has some sort of villainous force that is affecting the balance of power, similar to the villains in My Hero Academia. The heroes from My Hero Academia are transported to the Pokemon world to help. They could team up with Pokemon trainers and use their quirks in combination with Pokemon attacks to fight against the common enemy.
Common themes in 'godzilla x my hero academia fanfiction' include the concept of the extraordinary. Godzilla is an extraordinary being, and when combined with the already unique world of My Hero Academia, it creates a story full of wonder. The theme of cooperation also comes up. Since Godzilla is such a powerful entity, the heroes and even some villains might have to work together to deal with it. And there's the theme of the unknown. The characters in My Hero Academia are used to dealing with quirks, but Godzilla is something entirely different. So, they are faced with the unknown and have to learn about this new threat or power source, which makes for an exciting and often unpredictable narrative.
One possible plot could be that the heroes from My Hero Academia are transported to the Demon Slayer universe. They team up with the Demon Slayers to fight a new breed of demons that have quirks. Midoriya could learn some sword - fighting skills from Tanjiro and in return, Tanjiro could gain some knowledge about using quirks in combat.
One possible plot could be Sans using his teleportation ability (from his original game) in the My Hero Academia world to outwit villains during a big battle. For example, he could suddenly appear behind a villain like Stain and disarm him before he even realizes what's happening.
One possible plot could be that a powerful villain manages to release a virus that turns most of the population into mindless, powerful mutants. The remaining heroes, including the students of UA, have to not only fight these mutants but also find a cure. Deku could be leading the group, trying to figure out how to use his new powers in this chaotic situation.
A cool plot could be a training arc where the characters from My Hero Academia train with the Dragon Ball characters. Izuku could train with Gohan, learning about ki control and how to better use his power. Meanwhile, the Dragon Ball characters could learn about the importance of strategy in combat from the My Hero Academia heroes. And then, they could face a new villain that requires both their skills and powers combined to defeat.
The OC could be from a different country and has a very different approach to heroics. Mina is intrigued by this and tries to learn more about the OC's culture and way of using their quirk. As they interact more, they start to collaborate on hero missions. Their relationship develops as they overcome cultural differences and work towards a common goal of protecting people in the My Hero Academia world. This would allow for exploration of different cultures within the context of the story.
One possible plot could be that the characters from My Hero Academia are suddenly transported to the world of Blue Exorcist. They have to team up with the exorcists there to fight a new common enemy that has powers similar to the quirks in My Hero Academia but also has demonic abilities. For example, Izuku could learn some exorcist techniques from Rin and use his quirk in new and creative ways to combat this threat.
Another idea is that the Dragon Balls are discovered in the My Hero Academia world. Different factions in the My Hero Academia universe start competing to gather the Dragon Balls. This could lead to some unlikely alliances between heroes and villains as they all have their own reasons for wanting to make a wish with the Dragon Balls. The Dragon Ball characters could also be summoned by accident through the wish - making process, and then they get involved in the chaos of the My Hero Academia world.
A cool plot might be that the villains from My Hero Academia make a deal with Shang Tsung. In exchange for power, they bring the chaos of the Mortal Kombat world to their own. All Might then has to lead the remaining heroes, some of whom learn Mortal Kombat fighting styles, to stop the villains and the new threat from the other universe.