Well, in some cases, there could be minor characters from the original series who still interact with Harry on a limited basis. For example, Neville Longbottom might make an appearance. Even though Harry is alone for the most part, Neville could cross paths with him and have a brief conversation or exchange that has an impact on Harry's journey.
There could be his drill sergeant. A tough but fair character who pushes Harry to his limits. Maybe he has a name like Sergeant Thompson.
There could be Ron. He might be Harry's sidekick in the gambling adventures, either joining him in bets or trying to stop him when things get out of hand. Ron's loyalty would play a big part in their relationship within the story.
Ron might also be a main character. He could be initially skeptical of Harry's new powers but eventually come around and support him. He might even use his own skills, like his chess - playing strategy, to help Harry in battles or challenges related to his Dragonborn status.
Ron could also be a main character. He might initially be a bit confused or even jealous of Harry's new powers as an elf, but then come around to support him. His family, like the Weasleys, could also be involved, perhaps providing a different perspective on elves compared to the pure - blood wizarding families.
It's hard to say for sure without reading the fanfiction, but it's likely that Ron and Hermione are still important characters. They are Harry's closest friends in the original series, and their relationships with a prodigy - Harry would be interesting to explore. Maybe they have to deal with feeling overshadowed by Harry's new - found prodigious abilities.
There could be a new character introduced, a healer perhaps, who is intrigued by Harry's condition as a blind seer. This character could play an important role in trying to understand the nature of his seer abilities and whether there is a way to either enhance or control them. Another possibility is a rival seer who is jealous of Harry's unique situation and tries to undermine him throughout the story.
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.
Hermione could be one. She might use her vast knowledge of magic to help Harry figure out how to hatch the egg. Another could be Hagrid. He has a lot of experience with magical creatures, so he could offer advice on taking care of the egg and the soon - to - be - hatched creature.
The first years themselves would be main characters. There could be a shy first - year named Lily (not related to Harry's mother in this case). She might be the one who Harry takes a special interest in helping because of her insecurities.
Hermione and Ron would also be important characters. Hermione might be initially skeptical of Harry's newfound obsession with the Marauders. Ron, on the other hand, might be more easily swayed and join Harry in his attempts to be more like the Marauders.