There could be a human character, perhaps a teacher or a neighbor who notices the strange situation with the Cullens and the little girl. This character could add some extra drama as they try to figure out what's really going on with this family that seems a bit different. They might suspect the Cullens are up to something not quite normal and start to investigate, creating more storylines.
Well, perhaps the little girl has some special power that the Cullens notice. So they decide to take her in to protect her from those who might misuse her power. For example, she could have the power to predict the future in a very unique way that even the Cullens find fascinating.
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, maybe the little girl is a huge Avengers fan. One day, she gets into some sort of trouble, like being chased by bad guys who are after her unique abilities that she doesn't even fully understand yet. The Avengers find her and see her potential. Tony Stark might be the first to be charmed by her spunk and intelligence. So, they decide to take her in, protect her, and train her to be a part of their team in the future.
The Volturi could be the main antagonists. They are the powerful vampire leaders in the Twilight world and might not like the idea of a wizard like Harry being around the Cullens. They could see Harry's magic as a threat to their vampire - only world.
Obviously, the Cullens would be the main characters. Carlisle, as the patriarch, would play a crucial role in raising Bella. Esme would be like a mother figure to her. Edward would also be important, his relationship with Bella evolving from something different than in the original story. And of course, Bella herself is a central character.
In some stories, Bella could be the key to uniting different vampire clans. Since she's adopted by the Cullens, she might be seen as a symbol of a new way of vampire - human coexistence. This could lead to other vampires either trying to use her or trying to protect her from those who want to use her for their own gain.
I think it would be Spider - Man. He's closer to her age in a way compared to the other Avengers. He can understand her more easily, like the things she likes, the new trends, etc. And he has that friendly neighborhood hero vibe that would make the girl feel comfortable around him quickly.
One idea could be that the two broke girls, Max and Caroline, struggle at first to adjust to having a little girl in their lives. They might face financial difficulties in providing for her but find creative ways to make it work, like using their cupcake business to earn extra money for her school supplies.
In the fanfic, the little girl might influence their cupcake business. She could come up with new cupcake flavors that are a hit with the customers. This would help their business grow and also show how the addition of the little girl has a positive impact on their lives.