There's a possibility that it's an ancient magical being awakened by the emergence of the Death Stone. This being could have been dormant for centuries, and the power of the stone is what draws it back into the world. It might have a completely different view of magic and life compared to wizards, and its goals could be something that Harry and his friends find very difficult to understand, let alone oppose.
It could be a new dark wizard who has been secretly searching for the Death Stone. Maybe someone who was in the shadows during Voldemort's time and now sees an opportunity to rise with the power of this stone.
There could be a group of magical creatures who see the snake as an abomination or a threat to their kind. They would try to destroy the snake and Harry would have to protect it. These creatures could be from a part of the Forbidden Forest that has not been explored much in the original books and they have their own set of rules and magic which makes them a formidable antagonist.
Well, in such a fanfiction, Harry might face new and even darker challenges. The Death Stone could be a powerful and mysterious object with its own set of dark magic. Maybe it would attract dark wizards to Harry, putting him in constant danger.
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.
It could be a dark wizard who also wants to get to the secrets in the lost library. This wizard might try to stop Harry from exploring the library and getting its knowledge.
It could be Voldemort. Imagine he gets wind of Harry having this powerful Wolf Zord. He would stop at nothing to get it for himself, seeing it as a means to finally defeat Harry and take over the wizarding world completely.
Perhaps there's a member of the royal court who is secretly envious of Harry's power and tries to undermine him. This person could be plotting against Harry and trying to make the queen believe that he is a danger rather than an asset. They might spread false rumors or create false evidence to get rid of Harry.
It could be a new dark wizard who is jealous of Harry's ability to lift Excalibur. This wizard might want the sword for himself to gain ultimate power. He could start causing chaos in the wizarding world, trying to draw Harry out and take the sword from him. He might even team up with some of the Death Eaters who are also interested in the power the sword could bring.
Maybe the main antagonist is a dark force that has been slowly taking over the alliances from within. Harry notices this and decides to cancel the alliances to prevent the dark force from spreading further. This dark force could be something like an ancient evil that has awakened and is using the alliances as a way to gain a foothold in the wizarding world.
His best friends Hermione and Ron would surely be there to help. Hermione with her vast knowledge of magic could research about the egg, and Ron with his loyalty would stand by Harry's side in any danger related to the egg.