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.
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.
It depends on what you mean by 'real'. If you're asking if it's based on actual events, I'm not sure. Maybe it's fictional or inspired by some elements of reality.
Greta is partially based on a true story. The filmmakers likely incorporated real-life elements and added fictional elements to make the story more engaging and cinematic.
Alucard in such fanfiction is often depicted as extremely powerful yet loyal to Integra. He has his dark and mysterious allure, with his vampire abilities being a major part of his character. For instance, his shape - shifting and superhuman strength are often showcased in the stories.
They often have detailed settings. For example, in 'The Pillars of the Earth' by Ken Follett, the medieval building of the cathedral is described in great detail, making the historical period come alive.