Luna Lovegood could be an interesting main character in this fanfiction. Her unique perspective on magic might lead her to discover something about Harry's transformation that others miss. Hagrid would also be important. He loves Harry and would be very protective of baby Harry, using his knowledge of magical creatures to perhaps find a solution to the problem.
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.
Hermione could be a main character. She would probably be very involved in helping Harry with the baby, using her vast knowledge of magic to ensure the baby's well - being.
It could still be Voldemort. Even if Harry is turning to the darkside, Voldemort might see him as a threat because Harry has his own unique power. Voldemort might not want to share his control over the dark forces with Harry, so he would become the main antagonist in a fight for dominance in the dark world.
The baby could be the child of a minor character from the original series who was in hiding. For example, the baby might be the offspring of a wizard who was on the run from Voldemort's followers and unfortunately didn't make it, leaving the baby behind for Harry to find.
Obviously, Harry and Ginny would be main characters as the baby's parents. Their friends like Ron and Hermione would also play important roles. Ron could be the fun - loving uncle figure, always making the baby laugh with his silly jokes. Hermione would be the knowledgeable aunt, perhaps helping with the baby's magical education as it grows up.
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.
Dumbledore also frequently appears in these fanfictions. He plays a crucial role in Harry's life in the original series, and in baby Harry fanfictions, he might be shown as having a more hands - on approach in protecting and guiding the infant Harry. He could be the one making decisions about Harry's future and safety.
Some dark wizards or witches who are looking for power might see Harry as a tool or a target. If they think they can harness the power of a hollow - Harry, they would try to capture him. This could lead to many battles and challenges for Harry as he grows up in this strange new form.
Another possibility is that the wizarding establishment itself could be the antagonist. They might be afraid of Harry's new form and try to isolate or even eliminate him. After all, a hollow - like Harry could be seen as a threat to the normal order of the wizarding world, and they might not want to take any chances, so they would oppose Harry at every turn.
Neville Longbottom could be a main character. He has always had the potential for great things and in a world without Harry, he would be likely to step into the spotlight. He has the courage and the connection to Hogwarts that could make him a central figure in the fight against evil.