It could blend the two universes by having the characters interact. For instance, Levi Ackerman could meet Saitama. Levi would be impressed by Saitama's strength, but also notice his carefree attitude. Saitama, on the other hand, would be intrigued by Levi's skills. They could then team up to fight a common enemy, like a super - powerful Titan - like monster that is a threat to both of their worlds. This would bring out the best in both characters and create a unique dynamic.
Another concept could be a crossover where some of the Titans gain the power of One Punch Man's enemies. For example, a Colossal Titan-like being that has the regenerative abilities of some of the monsters Saitama has fought. The heroes from both universes would then have to team up to defeat this new threat. They would learn from each other's fighting styles and strategies.
Well, it could be about Saitama somehow ending up in the world of Attack on Titan. Maybe he's there to help the Scouts fight the Titans. He'd probably just one - punch all the Titans and make it seem like a really easy job, while the Scouts are left in awe of his power.
In a fanfiction of these two, the powers could be combined in a different way. For example, the Titans could have some of the characteristics of the monsters in 'One Punch Man'. They could be even more powerful and unpredictable. Meanwhile, the heroes from both universes could team up. Mikasa could learn some fighting techniques from Saitama, and Genos could be fascinated by the technology in the 'Attack on Titan' world. This would create a complex and exciting story with a lot of potential for character development and epic battles.
I think Eren and Saitama would also be a great pair. Eren has his complex motives and the power of the Titans, and Saitama is so simple - minded yet incredibly strong. They could interact in a really unique way, like Eren being all fired up to fight Titans and Saitama just being like 'Oh, that's nice' and then one - punching all the Titans. It would be a contrast of personalities and powers.
In a fanfiction, the ideologies of the two universes could also clash or merge. The Gundam universe often deals with themes like war, peace, and the morality of using advanced weaponry. In Attack on Titan, it's about survival, freedom, and the mystery of the Titans. A fanfic could explore how characters from both sides react to these different concepts.
One interesting twist could be that a Titan eats a character from One Punch Man and gains not only their physical abilities but also their memories. This Titan then starts to have an identity crisis and tries to rebel against the other Titans. It could lead to a situation where the heroes have to decide whether to kill this unique Titan or try to use it to their advantage.
Often, they blend the two worlds by having characters cross over. Like I said before, Saitama could be summoned as a Servant. Or the Fate characters could end up in the One Punch Man city.
It could blend the two universes by having Godzilla - like creatures be the result of dark magic experiments gone wrong in the Potter universe. Or perhaps the magic in the Potter world could be used to control Godzilla's power. For example, a powerful wizard might use a spell to direct Godzilla's atomic breath to defeat Voldemort.
The blend could happen through the use of prophecies. Since Apollo is associated with prophecy in Greek mythology, the daughter could have prophecies that are different from the ones in the Harry Potter series but still relevant to the overall battle against evil. This would introduce a new layer of mystery and foreshadowing. Also, the daughter's connection to the sun, as Apollo is the sun god, could be used to counter the darkness associated with Voldemort. Maybe she can create light shields or use sunlight - based spells.
It could blend the two universes by having a shared threat. Let's say an evil force is using both dark magic and the power of the Star Destroyer to conquer both the wizarding world and the galaxy. Harry Potter and his friends would then team up with some of the more heroic characters from the Star Wars universe, like Luke Skywalker. They would combine their skills, with the wizards using magic to counter the dark magic and the Star Wars characters using their blasters and ships to fight the Star Destroyer - related threats. This would create a really interesting dynamic where two very different worlds have to work together.