Well, a really interesting crossover is when the power systems of both universes collide. For example, when Fairy Tail mages try to understand Saitama's power which is seemingly limitless. It creates a lot of mystery and awe.
The crossover where Loki meets Lucy and they exchange magic knowledge is really interesting. Loki has his own set of powerful magic from Norse mythology, and Lucy has her Celestial Spirit Magic. The interaction between them as they learn from each other is captivating. For instance, Loki might teach Lucy some ancient rune magic, while Lucy shows Loki how to better communicate with spirits in the Fairy Tail way.
The crossovers that involve a combined guild or village are also fascinating. Imagine a place where Fairy Tail wizards and Naruto ninjas train together. In these fanfictions, there are often new rules and hierarchies created, and seeing how the different characters from both series interact within this new structure is very engaging. There are often rivalries and friendships that form in unexpected ways. For instance, Erza might become friends with Hinata because of their similar strong - willed natures, but at the same time, Natsu and Naruto could be competing to see who is the strongest in this new combined place.
A really interesting crossover is when the guilds in Fairy Tail have to face the threats from the Overlord world. For example, in some fanfics, Natsu and his team take on the powerful Floor Guardians. It's exciting to see how their magic stacks up against such formidable foes.
One of the most interesting crossovers is when the guild system of Fairy Tail is combined with the hierarchical structure of Overlord. For example, in some fanfics, the guild masters in Fairy Tail have to deal with the powerful lords from Overlord. It makes for some really intense power struggles and political intrigue.
The crossovers where the magic in Fairy Tail affects the Bakugan in unexpected ways are really interesting. For example, when a Fairy Tail mage's spell makes a Bakugan grow to an enormous size or gain a new power. It creates a whole new dynamic in battles and relationships.
A plot could involve a powerful enemy that is a threat to both the One Punch Man world and the Fairy Tail world. Maybe a being that can nullify magic and also has extreme physical strength. Saitama and some of the Fairy Tail members like Natsu, who is known for his never - give - up attitude, team up. Saitama's simple yet effective fighting style combined with Natsu's magic - based attacks could lead to an epic showdown against this common enemy.
A really interesting crossover concept is when the power systems merge. For instance, in a fanfiction, what if Naruto's chakra could be combined with the Sacred Gears in Highschool DxD? And Saitama's power could be something that disrupts or enhances these new combinations. You could have a story where the main character is a hybrid of a Naruto character and a Highschool DxD character, and they seek out Saitama to understand the true nature of power. This would involve a lot of exploration of how the different powers interact, like how Naruto's shadow clones could be used in a battle against Highschool DxD's devils, and how the devils' powers might affect Naruto's jutsu.
Another interesting pairing could be Tatsumaki from One Punch Man and Erza from Fairy Tail. Tatsumaki has powerful psychic abilities and a somewhat haughty personality. Erza, on the other hand, is a strong and disciplined mage. They could have a rivalry at first, both being very confident in their own powers. But as they face a common enemy together, they could learn to respect each other's abilities and form an unlikely alliance.
One Punch Man's heroes in the Marvel universe could also be a cool concept. For instance, Mumen Rider's sense of justice in a world full of super - powered beings like in Marvel. His simple yet strong - willed nature could inspire some of the street - level heroes in Marvel. And then there's Tatsumaki, her psychic powers could put her in some interesting situations with the likes of Doctor Strange, as they both deal with powerful and otherworldly forces.
Well, a great crossover idea is having the villains from One Punch Man enter the Naruto universe. For instance, Boros arriving in Konoha. The ninjas would be in for a huge shock as they've never faced an enemy as powerful as him. And Naruto would have to think of some really creative ways to defeat him. Also, the idea of Tsunade trying to figure out how to heal Saitama if he ever got 'injured' (which is highly unlikely) could be a humorous and interesting plot point in a fanfiction.