Voldemort is an obvious one. Given all the destruction and death he has caused, it's very likely that Harry would never forgive him. His actions led to the loss of many innocent lives, including Harry's parents.
Snape could be another. Even though in the original story there was more to Snape than met the eye, in this fanfiction, if Snape had done something truly unforgivable, like directly harming one of Harry's closest friends in a way that couldn't be undone, Harry would hold a grudge.
One possible reason could be the extreme betrayal he has faced. Maybe some characters he trusted deeply turned out to be working against him in ways that were unforgivable, like revealing secrets that put his life and the lives of his friends in danger.
Common characters he could meet include an au Ginny who is a powerful seer. There might be a Hagrid who is a tiny, timid man instead of the large, boisterous one we know. He could also encounter an au Sirius Black who is a Death Eater, which would be a shocking twist. Additionally, he may run into an au Professor McGonagall who is more of an artist than a Transfiguration teacher.
Well, in such fanfiction, Harry Potter could meet different versions of Dumbledore. In one universe, Dumbledore could be a dark wizard instead of the wise mentor we know. He may also come across alternate versions of Sirius Black. Maybe in some universes, Sirius is still in Azkaban for a crime he actually committed. Additionally, he could meet different versions of Luna Lovegood, some of whom might have no interest in the strange and wonderful things she loves in the original universe.
His friends from Hogwarts, like Ron and Hermione, are likely to help. Ron could bring his family's practical knowledge of wizarding homes and Hermione would contribute with her extensive research skills.
Given that it's a Harry Potter - themed fanfiction, Harry Potter would be a prime candidate for a main character. However, new characters might also be introduced in this fanfiction. Maybe there's a wizard or witch who has a special connection to this never - was castle. They could be guiding Harry and his friends through the mysteries of the castle, or perhaps they are the ones causing trouble within it. Hermione, with her vast knowledge of magic, would surely play an important role in exploring this strange new place as well.
One reason could be copyright issues. The creators of Harry Potter own the rights to the characters and story, and fanfiction often uses these without permission.
Hermione would be a great help. Her vast knowledge of magic and books could provide the necessary information for building the planet, like the right spells and magical theories.
He might meet Canadian witches and wizards. There could be a friendly wizard who shows him around the Canadian wizarding community.
He might meet international aurors. These are highly skilled wizards from different countries who are experts in dark wizard catching. They could teach Harry some new combat spells.
His friends from Hogwarts like Hermione and Ron would most likely support him. They have always been by his side through thick and thin in the original story, so in the fanfiction, they would continue to support Harry in his fight against the Ministry.