Well, it could be about Alucard and Seras teaming up to fight a powerful new vampire threat. Maybe they discover a group of vampires that are trying to take over the city and they use their combined powers to stop them.
In this fanfic, Alucard meeting his son could be a very emotional event. His son could be a product of a long - forgotten relationship. At the meeting, Alucard might see a lot of himself in his son, both in appearance and in personality. They could have a tense conversation at first, with Alucard trying to figure out what his son wants from him. As time passes, they might develop a mutual respect. His son could teach Alucard things about humanity and family that he had long forgotten. And Alucard could in turn pass on his skills and knowledge to his son, leading to a new chapter in both of their lives.
It could be an ancient and powerful vampire who has awakened from a long slumber. This vampire might see Alucard and Seras as a threat to his plans of ruling over all vampires.
Maybe there's a situation where a powerful artifact is being sought after. This artifact has the potential to cause chaos in both their worlds. Naruto and Alucard cross paths while both are on the hunt for it. At first, they compete for it, but then realize that they need to work together. Along the way, Naruto might try to get Alucard to see the value of friendship, and Alucard could show Naruto some of the secrets of the supernatural world that he doesn't know.
Naruto's chakra - based powers and Alucard's vampiric abilities could create some really cool interactions. For instance, Naruto's Rasengan could potentially be enhanced with Alucard's dark energy. Or Alucard could use Naruto's chakra to fuel some of his more draining attacks.
There might be a friend of Alucard's son. This friend could be a normal human who has no idea about Alucard's true nature. He could be the one who encourages Alucard's son to seek out his father and be there for him during the difficult process of getting to know Alucard.
There could be some of Harry's friends who might play a role. They could be the ones who encourage Harry to meet the Queen or who give him advice on how to behave in front of her.
Well, it could be about Sonic having an encounter with Queen Aleena. Maybe they team up to face a common enemy in the fanfiction. It might also involve Sonic learning something from Queen Aleena or vice versa.
Moana would probably be very curious about Elsa's ice powers. She might ask Elsa to show her how she creates ice sculptures. Elsa, on the other hand, would be interested in Moana's fearless attitude towards the ocean. They would start by sharing their different cultures. Moana would tell Elsa about the traditions of her island and Elsa would share about the life in Arendelle.