Hermione could be there. She would be really helpful in planning and organizing the island. For example, she might set up a system for categorizing all the magical knowledge and resources on the island.
Dumbledore might make an appearance in the fanfiction, even if it's just as a guiding spirit. His wisdom could be passed on to Harry as he builds and develops the island. Harry could have visions or dreams of Dumbledore giving him advice on how to make the island a place of harmony and learning for all wizards and witches.