Maybe the baby is a result of some sort of magical experiment gone wrong. And Harry Potter finds it in a hidden place where the experiment was carried out. The baby might have some strange and unpredictable magical abilities as a consequence.
It's possible that the baby is actually a time - displaced infant from the future. This would make for an interesting plot as Harry Potter tries to figure out how the baby got there and what it means for the future of the wizarding world.
Well, perhaps in this fanfiction, Harry Potter stumbles upon a lost baby in the wizarding world. He might then take it upon himself to find the baby's parents. Maybe he uses his magical skills, like casting spells to track the family. He could also seek help from his friends at Hogwarts, like Hermione who would surely be resourceful in finding clues about the baby's identity.
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.
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.
Hagrid would be a likely helper. He has a great knowledge of magical creatures and would surely know a lot about phoenix eggs. He could give Harry advice on how to take care of it.
Dumbledore could also be involved. He's extremely knowledgeable and powerful. He might have some secret knowledge about basilisk eggs. He could guide Harry on whether to keep the egg or destroy it, depending on the greater good of the wizarding world.
Hermione would most likely help. She's always been there for Harry, and with her vast knowledge from books, she could offer a great deal of advice on how to take care of the dragon egg. Ron might also be involved. Despite his initial fear of things, he has a brave heart and would probably stand by Harry's side.
The baby might be a key to a long - forgotten mystery. It could be related to an ancient pact between the gods. Percy finding the baby could be a sign that something big is about to happen in the world of the demigods. The baby's identity could be slowly revealed as Percy and his friends investigate. It could be a symbol of hope or a harbinger of doom, depending on the circumstances surrounding its appearance.
The baby could be an orphan from a war - torn area that Ruby Rose discovers. It might have no known family, and Ruby's discovery of the baby could be a turning point for her character, making her more empathetic towards the plight of the innocent in such situations.
It's possible that the baby's parents are from a rival pirate crew. In the 'luffy finds a baby fanfiction', this could lead to a lot of drama as Luffy has to decide whether to return the baby to its potentially dangerous family or keep it safe with his own crew. Or perhaps the baby's parents were victims of a powerful and evil force, and Luffy feels compelled to protect the baby from the same fate.
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.