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.
Ron could also be there. His loyalty to Harry and his own brand of courage would add to the story. Among the elves, there could be a young and adventurous elf who becomes friends with Harry. This elf could show Harry the hidden parts of the elven world and help him understand their culture better. And Dumbledore's spirit or some form of guidance from him might also play a role in the story, as he has always been an important figure in Harry's life.
Maybe Neville could make an appearance. He has grown a lot in the original series and his knowledge of plants and herbology could be useful in the elven realm which is likely full of unique flora. There could also be an elven wizard or mage who is an outcast from the council but has important knowledge that Harry needs to uncover in order to solve the problems the council is facing.