They could be part of a hidden threat in the wizarding world. Maybe they were sealed away long ago and are now being accidentally released. Their presence would cause chaos as they are so different from the usual magical creatures in Harry Potter.
One way the Weeping Angels could fit is through a time - travel aspect. Since they are related to time in their own 'Doctor Who' lore, they could be brought into the Harry Potter world through a time - travel accident. This could lead to all sorts of plot twists as the characters have to deal with the time - altering effects of the Weeping Angels while also dealing with their own magical problems.
One great fanfiction might be one where the Weeping Angels are introduced into the Hogwarts grounds. The Weeping Angels' time - freezing abilities could create really interesting scenarios with the magic of Harry Potter. For example, they could freeze some spells in mid - air, leading to a very different kind of battle or mystery for Harry and his friends to solve.
It could be a really dark and strange fanfiction. Harry would likely grow up in a very different and menacing environment. The Weeping Angels' nature of being stone when observed and deadly when not could lead to a lot of tense situations for Harry.
Harry would likely be very cautious. He's faced many dangerous creatures before, but the Weeping Angels are a whole new level of threat. He'd probably try to keep his friends safe while also trying to figure out how to stop them. Hermione would immediately start looking for a way to defeat them, researching in every book she could find. Ron would be on edge, ready to fight if necessary but also a bit nervous.
Harry Potter himself is a likely candidate. He has shown great courage and magical ability throughout the original series. His Patronus charm, for example, could potentially be used to fend off the King of Weeping Angels, as it represents his positive energy and strength of will.
Obviously, Harry Potter himself would be a main character. And the Weeping Angels would play a huge role too, acting as his guardians or captors depending on how the story is written. There might also be some Hogwarts professors who try to rescue Harry or understand his situation.
Gwendy can fit in as a new friend to the main characters. She could join Harry, Ron and Hermione on their adventures and offer a fresh perspective. Maybe she has a different approach to solving problems, like using her knowledge of Muggle technology in a magical way.
The Weeping Angels are a terrifying concept from 'Doctor Who'. In a Harry Potter fanfic, they could add a whole new level of horror. Maybe they could be introduced into the Forbidden Forest, lurking among the trees, turning any wizard or witch who looks at them into stone. It would be a really interesting crossover, changing the usually magical and somewhat safe environment of Hogwarts into a place full of lurking danger.
Harry could be seen as a strange but powerful being. His Gryffindor courage would be respected in Asgard. Hermione might be seen as a great scholar, like the Asgardian mages. Ron could be the loyal friend who has to adapt to the grandeur of Asgard.
Alucard could fit as a powerful outsider. His vampiric abilities like super strength, speed, and the ability to turn into mist or a bat would be very different from normal wizard magic. He could be a sort of anti - hero, helping Harry when it suits his own goals. For example, if there was a dark wizard using blood magic in a way that threatened Alucard's existence too.
Harry Hook often fits in as an alternate version or a new character added to the existing Harry Potter universe in fanfiction. He can bring new storylines and interactions.