One possible plot could be that a powerful dark wizard has set his sights on the Cullens for some reason, maybe to use their vampire powers for his own evil deeds. Harry, being the hero he is, senses this danger through his connection to the magical world. He then teams up with the Cullens, teaching them some basic wizarding defenses like Shield Charms. He also uses his own powerful magic, such as the Killing Curse in a defensive way if necessary, to save the Cullens from the clutches of this dark wizard.
Perhaps a dark wizard from the Harry Potter universe who has heard about the Cullens' unique vampire abilities and wants to harness that power for himself. He could be a follower of Voldemort who escaped the final battle and is now looking for new ways to gain power. So he targets the Cullens, thinking their powers can help him rule both the wizarding and vampire worlds, and Harry has to stop him.
In some fanfictions, Harry Potter might use his magic skills to help the Cullens. For example, he could use spells to protect them from their enemies. His knowledge of the magical world might also be useful in finding solutions to problems the Cullens face, like dealing with other supernatural threats.
One scenario could be that Harry uses his invisibility cloak to help the Cullens spy on their enemies without being detected. Another is that he could use the Time - Turner (if it exists in the fanfiction world) to go back in time and prevent something bad from happening to the Cullens.
Well, it could be that the Cullens and Harry Potter team up against a common enemy. For example, a dark wizard who has some sort of connection to vampires. Harry could learn about the Cullens' way of life and they could learn about the wizarding world. There might be a cultural exchange of sorts, with Harry showing them things like butterbeer and the Cullens introducing him to their vampire traditions, if they have any.
It could be really interesting. Harry might grow up with the Cullens' values of family and self - control. He could learn vampire - related skills perhaps in a magical way. For example, his ability to apparate might be enhanced by vampire - like speed. Also, the Cullens' diet of not harming humans could influence Harry's view on using magic for good and not for evil or harm.
Well, in such a fanfiction, Harry might gain new powers from the Cullens. Since the Cullens are vampires with unique abilities in the Twilight universe, Harry could learn things like super - speed or enhanced senses. Maybe he'd also have to deal with the vampire lifestyle and the rules of their coven.
Well, one possibility is that Harry would be introduced to a very different kind of family dynamic. The Cullens are vampires, so he'd have to adjust to their unique lifestyle. He might gain some new powers or abilities related to the vampire nature. Also, his relationships with the wizarding world would probably change drastically. He could bring some of his magic knowledge to the Cullen family, which would be quite interesting to see how they interact with it.
One common theme is the clash of two different worlds. The magical world of Harry Potter with its wizards, spells, and Hogwarts, and the vampire - centric world of the Cullens. Another theme is often the idea of friendship or alliance forming between the characters of the two universes.
He'd probably become more reserved. Growing up with the Cullens, who are often cautious and secretive, he'd pick up on those traits. His Gryffindor courage might be tempered with the Cullens' more calculating approach to life.