It could be a dark wizard who has a vendetta against the Weasley family. Since Harry is with Bill and Charlie, he gets caught up in the conflict. This dark wizard might see Harry as an easy target because of his shyness, thinking he can be easily intimidated.
In this fanfiction, perhaps Harry will be initially hesitant and quiet when meeting Bill and Charlie. He might be in awe of their more experienced and confident personas. Bill could offer Harry some words of wisdom regarding the wizarding world outside of Hogwarts. Charlie, on the other hand, might share stories about dragons which would both fascinate and intimidate Harry.
In some cases, it could be a corrupt Ministry of Magic official. This official might be trying to cover up some dark secrets related to magical creatures or certain magical areas that Bill, Charlie, and Hermione are investigating. The official would use his power and influence to try to stop them from uncovering the truth, creating a conflict where the three protagonists have to fight against the system to do what is right.
Well, perhaps they could go on an adventure together. Maybe they discover a hidden magical place in the Forbidden Forest. They might use their combined skills, Harry's courage, Bill's experience with magical creatures, and Charlie's knowledge of dragons to overcome some sort of danger or mystery.
They could be like brothers. Harry has a strong connection with the Weasley family already, and with Bill and Charlie, it could be a natural extension of that. They might look out for each other and support one another in any difficult situation.
There could be an ancient force that is awakened by the meeting of Harry and Hela. This force wants to destroy both of them because their combined powers could potentially stop it from achieving its goal of spreading chaos and destruction throughout all the realms. It would use all kinds of devious means to try and separate them and defeat them one by one.
It could be a dark wizard who wants to capture Nagini for his own evil purposes. Maybe he wants to use Nagini's connection to Voldemort to gain power.
One possible plot could be that Hermione meets Bill and Charlie during a magical creature research project. Bill's extensive knowledge of magical creatures from his work with Gringotts and Charlie's expertise from his time at the dragon reserve in Romania could complement Hermione's academic approach. They might start off a bit skeptical of each other, but as they work together, they come to respect and rely on one another, with Hermione learning a great deal from their practical experiences.
It could still be Voldemort. Even though the focus is on Harry and Nagini, Voldemort is Nagini's master and the main source of evil in the Harry Potter world. He would likely try to stop Harry from getting too close to Nagini or from finding out any secrets Nagini might hold.
It could be a dark wizard who is jealous of the bond between Harry and Lily. This wizard might try to break their connection and use Lily's power for his own evil purposes. Maybe he wants to control Lily's magic, which is known to be strong, to achieve domination over the wizarding world.
It could be Voldemort who somehow follows Harry into the realm of the Morrigan. His dark magic could disrupt the balance there and make him a common enemy for both Harry and the Morrigan.