He could have an adventure where he tries to broker peace between the vampires and werewolves. Given his noble status as the second son of a duke, he might have the influence and diplomatic skills to attempt such a feat. He could travel between the two camps, facing hostility from some members who are more set in their ways of the eternal conflict.
Perhaps he embarks on a journey to find a cure for vampirism or lycanthropy, believing that it could end the strife in the Twilight world. Along the way, he would encounter various challenges, from other supernatural beings who oppose his mission to the harsh terrains he has to cross. His duke - family - related resources could help him in some ways, like providing access to ancient texts that might hold clues.
One possible fanfic could be about the second son of a duke in the Twilight universe trying to find his place among the vampires and werewolves. Maybe he has a unique power that sets him apart from others. He could be caught in the middle of the eternal feud between the two groups, struggling to decide which side he should be on, or if he should try to create his own path.
He could have adventures involving other sea - related deities. Since Thetis is a sea - goddess, he might interact more closely with her kin. For example, he could be sent on a quest by Thetis to retrieve a powerful artifact from the deep sea that is guarded by a fearsome sea - monster.
He could have an adventure in the underworld. Maybe he has to rescue a friend or retrieve an important item that was lost there. With his divine powers, he might be able to fend off the creatures of the underworld like Cerberus.
I'm not sure specifically as there could be many different interpretations of a 'Twilight' fanfic about the second son. It might focus on a character who is the second son in the Twilight universe, perhaps exploring his relationships, powers, or his role within the vampire or werewolf clans.
He could have adventures that involve both the sea and matters of the heart. For instance, he might be sent on a mission to retrieve a magical item from a sunken city. But, along the way, he has to use Aphrodite - like powers to charm the sea creatures guarding the item. He could also encounter mermaids who are under a spell of jealousy, and he has to use his combined powers to break the spell.
He could have adventures in search of rare ingredients. Naruto, with his ninja skills, would be great at traversing difficult terrains to find ingredients that Toriko wants. They might go to places like the Gourmet World, where Naruto would face new and strange creatures.
They could go on a quest to find a magical flower that can make anyone speak in rhymes. Donkey would be excited about this as he loves to talk, and Dragon would be there to protect him from any danger on the way.
They could have an adventure in a mysterious forest. Hypno could use its powers to communicate with the forest Pokemon and find a hidden path that leads to a secret grove filled with rare berries. Serena could then use these berries to make special Pokeblocks for her other Pokemon.
They could have some really cool adventures. For example, they could team up to take on a powerful villain that is causing chaos in both their worlds. Naruto's ninja skills combined with Batman's detective skills and gadgets would make for an unbeatable team. They might also go on a journey to find a way to send Naruto back to his own world if that was the case, encountering all sorts of strange creatures and obstacles along the way.
He could go on a journey to save Christmas. Maybe some dark magic has affected the North Pole and it's up to Harry to use his wizard skills and his newfound knowledge of the Christmas magic to fix things. He'd have to battle against dark elves or some sort of anti - Christmas spirits.