It could be a dark wizard or sorcerer who wants the dragon egg for its power. They might try to steal the egg from Bella.
In some fanfics, it could be an ancient evil that has been waiting for the dragon egg to hatch. This entity believes that the dragon can be used to bring about the end of the world or at least cause chaos. So it tries to take the egg from Bella. It might also have minions or servants who do its bidding and make it even more difficult for Bella to protect the egg.
Perhaps a group of hunters who capture rare creatures. They see the dragon egg as a valuable trophy and will stop at nothing to get it from Bella. In this fanfic, they would be the main obstacle for Bella to overcome.
In the 'bella finds a dragon egg fanfic', there are many possibilities. The dragon egg could be the key to unlocking a hidden magic in Bella's world. As she takes care of the egg, she might learn new spells or abilities herself. She could form a deep bond with the dragon once it hatches. The fanfic could also explore how the discovery of the egg changes Bella's life. For example, she might become more responsible and brave as she has to protect the precious egg and the young dragon that will emerge from it. It could also lead to her getting involved in a larger conflict between different factions who either want to protect or use the dragon for their own gain.
Maybe Kakashi. He's very experienced and wise. He could share some of his knowledge about strange things he has encountered before. He might help Naruto understand the significance of the egg and guide him on what to do with it, whether it's to keep it safe or to find its rightful place.
The White Walkers could be the main antagonists. Since Jon Snow has found a dragon, they might see it as a threat to their plans of taking over the Seven Kingdoms. They would try to kill the dragon and Jon Snow to ensure their dominance remains unchallenged.
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.
Daenerys Targaryen could be one to help. Since she has experience with dragons, she could give Jon Snow advice on how to care for the egg and hatch it.
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.
In a fanfic scenario where Arya finds a dragon egg, it could change the entire course of the Game of Thrones world. Maybe she would try to hatch it in secret, using her skills of stealth. If she succeeded, she could become a powerful force. She might use the dragon to protect her family or seek revenge on her enemies. However, she would also have to face the challenges of hiding and training a dragon in a world full of political intrigue and danger.
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.
Thor could be one. Maybe he thinks the dragon egg is too dangerous in Loki's hands and wants to take it away for the safety of the realm. Or Odin, who always wants to control powerful artifacts. He might see the dragon egg as something that should be in his possession.
A secret organization could be the antagonist. They might see Bella as a threat to their hidden agenda. For instance, if she has stumbled upon some information about their illegal activities, they lock her in a room to keep her silent and prevent her from sharing what she knows with the outside world.