One of the main characters could be Hagrid. He has a connection to many magical creatures and his knowledge could be valuable in helping Harry when he's mistaken for an elfling. Also, there could be an elven character, let's say Elara, who befriends Harry. She is a young elven maiden who believes that Harry is a special elfling. She helps him adapt to the elven lifestyle and protects him from those who might want to harm him because of the confusion. Neville could also be involved as he is Harry's friend and would surely want to help him out of this strange situation.
Dobby could be a main character. Since he is an elf, he could play a crucial role in helping Harry deal with being mistaken for an elfling. He could be the one to guide Harry through the elven world and explain things to him. Another character could be Hermione. She would surely be trying to research ways to reverse the situation or understand why Harry is being mistaken for an elfling in the first place.
Hermione could be there. She would be eager to study the different magics and books in Tamriel. Her intelligence would help Harry navigate the new world.
Hermione could be a main character. As a very intelligent witch, she might assist Harry in his teaching, like helping him prepare complex lessons or providing additional knowledge to the students. For example, when Harry is teaching about ancient spells, Hermione could offer insights from the books she has read.
Hermione could be one. She could come to Degrassi to help Harry. She'd be really good at handling the academic side of things there, like helping students with their studies. And also, she could use her magic knowledge to figure out any magical mysteries.
A possible plot in 'Harry Potter mistaken for Elfling fanfiction' is that the magical community outside of Hogwarts mistakes Harry for an elfling. They assume he has elven heritage due to some old prophecy. This leads to him being involved in elven politics and power struggles. Harry has to use his wits and the little he knows about the elves to survive. Along the way, he makes friends with some elves who help him understand his situation better and eventually he clears up the misunderstanding.
There could be the leader of the coven. This person might be an old and powerful witch or wizard who has been in the coven for a long time. They would be the one to guide Harry into the ways of the coven.
Megatron could also be a major character. Maybe he views Harry Potter as a threat to his power and plans. He could try to either destroy Harry or use his unique abilities for his own evil purposes. And then there could be Bumblebee, who befriends Harry and helps him communicate with the other Autobots, since Bumblebee is often good at making connections.
Hermione could be a main character. She would use her intelligence and vast knowledge of magic to help Harry plan and build the city. Her skills in research and spell - casting would be invaluable.
The elves themselves would be major characters. There could be an elven leader, perhaps an old and wise elf who is the head of the council. Also, Hermione might be involved as Harry's friend and often his partner in adventures. She could bring her vast knowledge of magic and research skills to help Harry in dealing with the council of elves.
Hermione would surely be a main character. She could be a fellow teacher, perhaps teaching Transfiguration or History of Magic. She would be the one Harry goes to for advice on teaching methods or when he has a difficult student. Another character could be Ron. He might visit Harry at Hogwarts and end up getting involved in some of the school's mysteries or helping Harry with a particularly difficult Quidditch - loving student.
Ron would also be a main character. He might start off by making fun of Harry's braces, but then he would stand up for Harry when some of the Slytherins start to bully him because of it. Ron could also try to use the braces to his own advantage in some comical ways, like using them as a slingshot for Bertie Bott's Every Flavour Beans.