Definitely James Potter is at the center. But his friends like Sirius, Lupin, and Peter are likely to have important roles. Their group dynamic could be really different with Harry's influence. Lily is also a must - as his future wife and the mother of Harry (in the original timeline). Hagrid could also be important. Maybe Harry - as James - would involve Hagrid more in his plans to change things. And Bellatrix Lestrange could be an interesting character to include. Harry, now James, might try to prevent her from going down the dark path she took in the original story.
Well, first off, James Potter is the central character now. But don't forget about Snape. Given Harry's past experiences as himself, his new identity as James could lead to a very different dynamic with Snape. Lupin would also be important as one of James' friends. And Voldemort, of course. Since Harry's whole goal might be to stop him in a new way, Voldemort would be a major presence in the story. Also, Petunia might have an unexpected role, as Harry now has a different perspective on his Muggle family.
Harry Potter himself, of course. He has always been at the center of things in the wizarding world. His actions and decisions would likely have a great impact on any change that occurs.
Neville could be another character. He might look up to Harry even more in his dragon form, seeing him as an even more powerful protector. Hagrid would be excited to have a dragon - Harry around. He could help Harry understand his new dragon nature. And some of the Slytherins, like Draco, might be jealous or scared of Harry's new form.
Obviously, Petunia in the form of reborn Harry would be the main character. Vernon and Dudley Dursley would also be important as they are part of the family. And we can't forget about the wizarding world characters like Dumbledore. He would surely notice some changes in Petunia and might start to investigate. Also, there's a good chance that Hagrid would be involved in some way, perhaps bringing some news or event that would set the story in motion for Petunia!Harry.
Obviously, Harry Potter and Regulus Black are the main ones. Harry as the boy who lived, and Regulus as his guardian, would be central to the story. Their relationship would be explored deeply.
Well, Hagrid would be a great addition. He could bring in some friendly magical creatures for the preschoolers to see, like baby owls or small unicorns. The kids would be thrilled to meet them.
Hermione Granger would surely be one of the first to support him. She's always been by his side, helping him with research and providing intelligent advice. Ron Weasley would also be there, being Harry's loyal friend through thick and thin. Neville Longbottom could be another supporter. He's grown into a brave and capable wizard, and he would stand with Harry in his new role. Dumbledore, if he were still alive in the fanfiction, would likely offer his wisdom and guidance to Harry as he takes on this new and important role.
The younger students at Hogwarts are quite likely to read Harry Potter fanfiction. They are at an age where they are really into the wizarding world and would be curious about different interpretations. Students like Colin Creevey, who is a big fan of Harry, might be eager to read fan - created stories about his hero. Also, the members of Dumbledore's Army, such as Luna Lovegood and Neville Longbottom, seem like the types who would be open to exploring fanfiction.
Lily Potter would also be involved. She was James' wife, and if James saw something in the future that affected their family, he would surely involve her. Also, Dumbledore might be in the story. He was always a key figure in the wizarding world, and James might seek his advice on what to do with his visions of the future.
Well, a plot might involve Harry - now James - trying to change the fate of his family. He could focus on protecting Lily in a different way than James did in the original story. Maybe he'd use his knowledge of magic from his past life to create new spells or defenses. Also, his relationship with Snape could be very different. Since he was Harry before, he might understand Snape's feelings better and try to make amends or change the animosity between them.
One great 'Harry Potter James Potter' fanfiction is 'The Marauders' Era'. It delves deep into the friendship of James Potter and his friends. It shows their adventures at Hogwarts, like their pranks on Snape. The characters are well - developed and it stays true to the original Potterverse feel.