A plot could involve Baldur being sent to the realm of God of War by Odin to spy on Kratos. However, Baldur is intrigued by Kratos' strength and his relationship with his son, Atreus. Baldur starts to question his own loyalty to Odin. Meanwhile, Kratos is suspicious of Baldur but also sees a bit of his own past self in Baldur's blind obedience to a cruel father figure. Eventually, they work together to resist Odin's control over Baldur.
Perhaps in the fanfiction, there could be a magical accident that fuses the two universes. Baldur and Kratos find themselves in a strange new world that is a mix of their own. They have to rely on each other's skills to survive. Baldur's invulnerability and Kratos' combat experience make them a formidable team. Along the way, they meet characters from both universes and have to navigate political intrigue and battles, all while trying to find a way to separate the two universes again.
Odin could be the main antagonist. He is a powerful and manipulative figure in Norse mythology. He might see Kratos as a threat to his power and send Baldur to deal with him. But in reality, Odin could be using both of them for his own dark purposes, like obtaining some powerful artifact or spreading chaos in the realms.
One possible plot could be that Kurosaki Ichigo from Bleach somehow gets transported to the world of God of War. He might team up with Kratos. They could face off against a new threat that combines the Hollows from Bleach and the monsters of God of War. For example, Hollows with the strength and brutality of the Cyclops. Ichigo's Zanpakuto skills would be interesting to see combined with Kratos' brute force combat style.
Another possibility is that there is a new threat that emerges from the combination of the two universes. Some sort of evil entity that has the powers of both the Naruto universe, like the ability to use chakra, and the dark magic from the God of War world. Naruto and Kratos would have to learn to trust each other quickly in order to stand a chance against this new foe. They might start off at odds with each other due to their different backgrounds and fighting styles, but eventually realize that they need to work together.
One possible plot could be that Kratos, from God of War, somehow ends up in the Assassin's Creed universe. He teams up with an Assassin, perhaps Ezio. Together, they face a common enemy that threatens both their worlds. This enemy could be a powerful Templar who has acquired some of the dark magic from Kratos' world.
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.
One possible plot could be Goku, in his Saiyan tenacity, challenges the God of Destruction to a friendly spar. The God of Destruction initially scoffs but is soon impressed by Goku's power. As they fight, their powers start to blend, creating new energy forms that threaten to disrupt the balance of their universes. They then have to team up to contain this new energy while also learning from each other's fighting styles.
One idea could be that Ben Carmine gets transported into an alternate dimension where the Gears of War universe has some strange new threats. He teams up with the Gears there, using his skills from his own world to help fight off the Locust horde. Maybe he brings some unique weaponry or tactics that the Gears haven't seen before.
One possible plot could be that Naruto and Percy Jackson are somehow transported to a realm where the gods of both their universes co - exist. They are initially confused but quickly realize they need to work together. Naruto's ninja skills and his connection to the chakra (which could be seen as a similar energy to the magic in Percy's world) combined with Percy's control over water and his demigod powers would make for an exciting adventure. They could be tasked with a mission by the gods, like retrieving a powerful artifact that has been stolen and could disrupt the balance between the two worlds.
One idea could be a crossover with Norse mythology outside of the God of War series. For example, Kratos could meet Thor from the original Norse tales. Another could be a crossover with Greek and Roman mythology, where Kratos encounters some of the forgotten or lesser - known Roman gods. Maybe a crossover with a popular fantasy world like Middle - earth, and Kratos has to face off against some of the powerful beings there.
A crossover with another popular action - adventure game like 'The Witcher'. Geralt and Kratos could meet in a world that is a blend of their two universes. They might initially be at odds due to their different moral compasses, but then find common ground in fighting a powerful, otherworldly enemy that neither could defeat alone. This would create an interesting dynamic between their combat styles and personalities.