The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.
Well, first of all, there's Triton, his son. Triton was like his father in some ways as he also had a connection to the sea. Then there's Amphitrite, Poseidon's wife. She was a sea - goddess. And, as mentioned before, Athena was very much related in the story of the competition for Athens. Also, his many children, like Polyphemus the cyclops, are important characters in relation to him as they were part of his story of having numerous offspring.
Amphitrite is very important. She is Poseidon's wife. Their relationship is crucial as it gives context to Poseidon's family and his role as a married god in the mythological world.
In 'Poseidon the First Stories', there might be stories about his relationships. He has many affairs and children. His relationship with Amphitrite is a significant part. He had to court her, and their union led to various other stories, like the birth of some of his important children who had their own roles in Greek mythology.
Deku might be completely shocked at first. Since Bakugou has always been so aggressive towards him, he would never expect such a confession. He might just stand there, mouth agape, not knowing what to say.
His friends could come to the rescue. Bakugo, for instance, might use his explosive power to break into Toga's hideout. He's always been competitive with Deku, but he also cares about him deeply.
In 'deku meets future deku fanfiction', perhaps the future Deku warns the present one about upcoming threats. He could tell him about villains he'll face in the future and how to defeat them. This would give the present Deku an edge in his battles and also in his growth as a hero.
Well, obviously Deku in two forms - the younger and the older version are the main characters. The younger Deku is the one who is still growing and learning, while the older Deku has more experience.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfictions across various fandoms, including those related to Poseidon. Just use the search function and type in 'Poseidon' to find relevant stories.
Perhaps there is a crossover event in the fanfiction. Some sort of magic or powerful artifact causes the Naruto universe and the world of Poseidon to collide. Naruto, being the adventurous and powerful character he is, gets involved in this chaos and ends up having a special relationship with Poseidon, either as an ally or having to face him as an adversary.