It could be a passionate Potterhead who has a great imagination for creating new adventures for Harry. There are many fans out there who love to write their own take on the Harry Potter universe.
There are also the possibility that it could be a group of fans working together. They might each bring different ideas about how Harry would interact with the magic book, such as one focusing on the spells in the book, another on the historical significance of the book, and they combine their efforts to create a really interesting fanfiction.
Well, it could be that the magic book is sentient and starts to communicate with Harry. It could offer him advice on how to defeat Voldemort in a completely new way, like revealing a long - forgotten prophecy.
It could be someone who also wants the magic book. Maybe a rogue ninja who believes the power in the book can make him the most powerful in the world.
The genie might be a transformed wizard. Some powerful dark magic could have turned a wizard into a genie as a form of punishment. When Harry finds the lamp, he unknowingly releases this wizard - genie. This genie - wizard could have knowledge of magic that has been long forgotten, which could be very useful for Harry in his fight against Voldemort.
The baby could be the child of a minor character from the original series who was in hiding. For example, the baby might be the offspring of a wizard who was on the run from Voldemort's followers and unfortunately didn't make it, leaving the baby behind for Harry to find.
It could be about Harry Potter somehow being transported to the Avatar world. Maybe he discovers he has bending abilities and has to learn to control them while also dealing with the political and social issues in the Avatar universe.
It could be an evil dark wizard. This wizard might have created the mask with the intention of using it for some nefarious purposes, like taking over the wizarding world. But before he could use it, the mask was lost or hidden, and now Harry has found it.
It could be a dark wizard who wants to get his hands on the lightsaber for its power. He might see it as a new tool to use in his evil plans, like using it to break into Gringotts or to fight off any who oppose him.
It could be a dark wizard who also has an interest in the snake. Maybe he wants to use the snake's powers for evil purposes, like opening a portal to a dark dimension or controlling other magical creatures. So he becomes the main obstacle for Harry as he tries to protect the snake.
Well, it depends on the context. If he's in a place where he has some free time, like in his dormitory, he could immerse himself in the fanfiction. He could share it with his friends too, especially Hermione who would probably be interested in analyzing the old magic described in it. And who knows, it might even inspire him to try out some new magical ideas in a safe way, perhaps during one of their magic practice sessions.
Maybe it could be an ancient magical being like a centaur. Centaurs are known for their knowledge of the stars and ancient magic. They might take Harry under their wing and teach him old magic that has been passed down through their generations.