Maybe some of the Weasley twins would be involved. They could see the potential for some pranks with the cars. For instance, they could create a car that shoots out confetti or makes funny noises every time someone starts it. And Neville could be there as well, learning from Harry and slowly getting more confident in his own abilities as he helps with the car - working tasks.
Ron could be there too. He might be more interested in the cool features of the cars, like the sound systems or the design. He could be the one who tries to convince Harry to add some fun elements to the cars they work on. And Hagrid might also show up, being all amazed by these strange Muggle contraptions and accidentally breaking something while trying to figure out how they work.
In this fanfiction, Neville could also play a role. He has grown into a brave and capable wizard. He might use his skills with plants to create some sort of magical plants that could help, like plants that can grow quickly to block the path of the iceberg or to repair any damage to the ship. And Luna could be there with her unique perspective, perhaps she could sense the danger in a different way and help Harry in his mission to save the Titanic.
There could be some new characters created specifically for this fanfiction. For example, an old wizard who lives near the fishing area and gives Harry some tips on magical fishing. Or perhaps a mermaid who tries to play tricks on Harry while he's fishing. Hagrid might also be involved, as he loves all kinds of magical creatures and fishing could be a new adventure for him too. He could bring along some of his strange pets to the fishing trip.
Hermione could be involved. She might be the one who comes to check on Harry because he's been in the shower for a long time. She could be worried that something's wrong, like if a dark spell has been cast on him. And when she knocks on the door, Harry has to quickly figure out how to respond while still being in the shower.
Dumbledore could make an appearance. He might be the one who gives Harry the idea to become a pilot in the first place or provides some ancient magical knowledge about flying that helps Harry in his new career. Also, the Weasley twins could be involved. They could create some magical gadgets for the plane, like a device that makes the plane do cool stunts.
Well, perhaps Harry uses magic to fix cars in a really unique way. Like he might use a spell to make the engine parts float into place instead of using tools.
There could also be new characters introduced. Maybe other magizoologists from different parts of the wizarding world. They could have different approaches and beliefs about magical creatures. Some might be rivals, trying to outdo Harry in the discovery and study of new species, while others could be mentors, guiding him in the complex world of magizoology.
Neville could be part of it too. Maybe he sees Harry's training and gets inspired to improve his own magical abilities. He could learn from Harry's experiences under Sirius's training. And the Weasley twins might show up to add some mischief and perhaps even create some magical gadgets to aid in Harry's training.
Obviously, Harry Potter himself. Then there could be Hermione and Ron, who would be his friends and help him deal with the revelations of his parentage. Dumbledore would also be there, guiding Harry as he tries to understand his connection to Ariel. And perhaps some sea - based characters like mermaids or sea wizards who come to Hogwarts to interact with Harry.
On the Hogwarts side, Hermione and Ron would surely be there. They could be quite bewildered by Harry's newfound connection to Olympus. And Dumbledore, he might have some knowledge about the connection between the two worlds and could guide Harry in some way. From the Olympus side, Athena could also be involved as she is the goddess of wisdom and might see potential in Harry.
Obviously, Harry Potter himself is the central character. Hermione and Ron would most likely be involved too. They would probably try to find a way to get Harry out of Azkaban. Dumbledore might also play a role, perhaps guiding Hermione and Ron from behind the scenes or even trying to influence the wizarding court to re - evaluate Harry's case.