Ron would also be an important supporting character. He could be a bit confused by Harry at first but then grow to be his loyal friend. Ron might provide a more 'normal' wizarding - world view that contrasts with Harry's 'baby - adult' view. He could also be the one to get Harry into some light - hearted mischief, despite Harry's more serious side.
Hermione would be a great supporting character. She could be intrigued by Harry's strange behavior for a baby. She might try to study him like she does with her books, constantly observing and trying to figure out how he has such adult - like qualities. Their interactions would be interesting as Harry could offer her different perspectives on magic that she hadn't thought of before.
Dumbledore would surely play a major supporting role. He would be one of the few who could truly understand the significance of Harry's situation. Dumbledore would be guiding Harry in a different way than in the original story, respecting his 'adult' knowledge while still protecting his innocence as a baby. He could be a mentor to Harry in a more complex relationship than in the original series.
Voldemort would surely be a main character. With Harry dead as a baby, Voldemort's actions and how he further consolidates his power would be a major part of the story.
The iCarly crew could all be supporting characters in different ways. Spencer could be the cool uncle who builds amazing things for the baby. Tasha could be there to offer maternal wisdom to Carly. And the fans of iCarly in the fanfiction could also play a role, like helping out with a fundraiser for the baby's future education or something similar.
Haymitch could be a supporting character. He might offer some advice to Peeta based on his own experiences, even if it's a bit rough around the edges. Also, Effie could be there, fussing over Katniss and trying to make the whole situation as proper as possible in her own unique way.
Definitely Hermione. She would be the one to encourage Harry to keep going and not let his parents' disowning define him. Neville could also be a great support. He has his own experiences of being underestimated, and he could share his strength with Harry. And Luna. Her unique perspective on life and magic could give Harry new ways to think about his situation, and she would offer him her unwavering friendship.
Hermione and Ron would definitely be important supporting characters. Hermione could be Ginny's confidante, sharing her wisdom about family planning and motherhood. Ron could offer Harry some brother - in - lawly advice, perhaps with his own unique take on parenting based on his large family experience. The Weasley family as a whole would also be there. Molly Weasley would be overjoyed at the idea of a new grandchild and would likely be full of advice and love. Arthur Weasley might be curious about how magic could be incorporated into raising the baby. Luna Lovegood could also be a great addition, offering her dreamy and often profound insights on new life and the future.
Fleur would definitely be a major supporting character. As Bill's wife, she would play an important role in Harry's new family life. She could teach Harry some French magic or wizarding traditions. The other Weasley siblings could also be supporting characters. They might visit often and continue to be like Harry's extended family. Charlie could come and share his dragon - taming stories with Harry, adding more excitement to Harry's new life.
Ron could also play an important role. He might be initially scared or confused by Harry's Animagus form but later becomes used to it. Ron could use his family's knowledge of magic, especially things related to magical creatures, to help Harry adapt. For instance, his family's old tales about Animagi could provide valuable insights.
Robin could be another good one. He could act as a sort of big brother figure to the baby daughter. He could play with her, teach her some basic self - defense moves (in a very kid - friendly way), and help Batman keep an eye on her.
Megatron could also be a major character. Maybe he views Harry Potter as a threat to his power and plans. He could try to either destroy Harry or use his unique abilities for his own evil purposes. And then there could be Bumblebee, who befriends Harry and helps him communicate with the other Autobots, since Bumblebee is often good at making connections.
The main characters would obviously be the five Avengers. It could be Iron Man, Captain America, Thor, Black Widow, and Hulk. And of course, the baby would be a central character as well. Maybe the story could focus on how each of the Avengers interacts with the baby differently. For example, Iron Man might use his technology to create baby - friendly gadgets, while Thor might tell the baby stories of Asgard.