Maybe there are some villains. For example, an enemy general who is trying to stop Harry from achieving his goals with the tank. There might also be some side characters like a mechanic who helps Harry fix the tank when it breaks down in the middle of his journey.
There could be a mentor figure who has experience with tanks and teaches Harry the ropes. This person could be an old soldier who has seen many battles. Also, there might be civilians in the area where Harry is driving the tank, and their reactions to the tank and Harry could play a part in the story.
Maybe Harry is in a post - apocalyptic world and uses the tank to fight off some evil forces.
Hermione would probably be one of the main characters. As Harry's friend, she could be a teacher at Hogwarts, perhaps the new Transfiguration teacher. She would offer her intelligence and knowledge to support Harry in running the school.
The 'bro' could be another important character. It might be a friend of Harry's like Ron or Hermione, who has been transformed into a 'bro - like' figure for the purpose of this fanfic. There could also be a healer or a wizard who is an expert in the strange magic that caused Harry to give birth. Maybe Dumbledore could also make an appearance to guide Harry through this difficult situation.
Luna could be in this fanfiction. She would probably have a very unique perspective on Harry's actions. She might see it as a sign of Harry breaking free from the norms that have been confining him. And Neville could also be there, perhaps being one of the few who understand Harry's reasons on a deeper level.
Another character could be a cologne - making elf. In the wizarding world, there could be elves who specialize in making colognes. This elf could have a special bond with Harry. It could be that the elf was tasked with making a special cologne for Harry's protection from the moment he was born. And as Harry grows, the elf helps him understand the true power of the colognes he has been surrounded with all his life.
Hermione could be one. She might follow Harry to America to continue their adventures together. She could help Harry adjust to the new place with her intelligence and knowledge.
There could be a young and brave goblin warrior who is eager to prove himself in the fight alongside Harry. And perhaps there's a goblin shaman or mage who has special powers and uses them to help in the battles. These goblin characters would each contribute in different ways to the story.
Ron could also be in it. He might initially not believe that Harry is hiding his intelligence, but as the story progresses, he could start to notice little things that Harry does which show his real intelligence. Ron's loyalty to Harry would then play a role in how he reacts to this discovery, either helping Harry keep it a secret or getting frustrated with Harry for not sharing his intelligence more freely.
There could be Firenze, the centaur. Centaurs are known for their wisdom about the forest and its creatures. He might be able to guide Harry to the unicorn and provide some herbal remedies or magical advice for the healing process. Also, Hermione could be involved. She would surely do a lot of research in the library to find the best way to heal the unicorn.
Hermione could be there. She might be the one explaining the significance of Beltane to Harry. Ron could also be present, probably more interested in the food at the Beltane celebration.