Another plot could be that an ancient prophecy in the One Piece world foretells of a 'Luffy God of War' who will bring about a new era. Luffy then has to fulfill this prophecy while dealing with his own doubts and the expectations of his crew and the people he has met on his journey.
Perhaps in this fanfiction, Luffy is transported to a realm where he has to prove himself as the God of War. He could face challenges that test not only his physical strength but also his leadership and strategic thinking, which are also important aspects of being a God of War.
One idea could be exploring Kratos' past in more detail. For example, his early days as a Spartan warrior before the events of the main games. Maybe he had a comrade in arms that he lost, and in the fanfiction, this comrade's spirit somehow resurfaces and has a message for Kratos. Another plot could be Kratos having to face a new type of enemy that is not from Greek or Norse mythology, but from a completely different culture's pantheon, like the Egyptian gods. This would force him to adapt his fighting style and strategies.
One idea could be that Kratos and Freya team up to face a new threat from the Norse underworld. They combine their combat skills and knowledge to overcome hordes of demons.
A great plot idea could be Percy Jackson being transported to the world of God of War and having to adapt to its harsher rules. He'd have to learn to fight without relying solely on his water powers, as the enemies there are more fierce and less predictable. Maybe he gets mistaken for an enemy by Kratos at first and has to prove himself.
Another plot idea is that Kratos somehow ends up in the Naruto universe. He could be mistaken for a powerful rogue ninja at first. His brute strength and combat skills would make him a force to be reckoned with. He might get involved in the conflicts between the ninja villages, and his no - nonsense attitude could either cause more chaos or actually help bring about peace in a rather unexpected way.
One idea could be Kratos arriving in the RWBY world and getting involved in the fight against the Grimm. His combat skills and Spartan rage would be a huge asset. Maybe he teams up with Ruby and her friends, teaching them some of his more brutal combat techniques.
A cool god of war fanfiction could be set in an alternate universe where the gods have different personalities. For example, Zeus could be a benevolent figure instead of the antagonist. Kratos might have to deal with this new dynamic and how it affects his own journey of vengeance. You could also write about a fanfiction where Kratos has to train a young apprentice, passing on his skills and wisdom.
Maybe they find a strange fruit that has the power to temporarily swap their abilities. This leads to some comical and also interesting situations. Luffy tries to use Robin's powers to reach for things in a more elegant way, and Robin attempts to use Luffy's stretching ability to explore hard - to - reach places. It also makes them understand each other's perspectives better.
Perhaps there could be a plot where a magical artifact is lost and it has the power to change the balance in both universes. Naruto and Fem Luffy both hear about it and start their own quests to find it. But then they realize that working together is the best option. They face many challenges, like different types of enemies and environmental obstacles, as they search for the artifact.
One idea could be that Luffy joins the Fairy Tail guild and his rubber powers amaze everyone. He might participate in a guild - wide competition and use his unique abilities to win in unexpected ways. For example, stretching to reach far - off targets or bouncing around the arena to dodge attacks.
A possible plot is that Luffy discovers a time - traveling device at Marineford. He then uses it to go back in time briefly to prevent some of the worst events from happening, like Ace's capture. Or perhaps Luffy has a dream where he is given advice by a long - dead pirate on how to win the battle at Marineford. This advice then helps him make better strategic decisions during the real battle. He could also meet a mysterious character at Marineford who has the power to heal all the wounded, which changes the whole course of the battle.