Another possibility is some of the other vampires in the Twilight world. They might not like that Edward has left Bella for a wizard. They could see it as a betrayal of their kind and try to get Edward back to Bella or cause trouble for him and Harry.
Maybe Voldemort. If the fanfiction combines the Twilight and Harry Potter universes, Voldemort might see Edward as a threat because he is a vampire and could be allied with Harry. So he tries to drive a wedge between them and make Edward leave Harry.
Edward would probably be very confused at first. He would try to understand why Bella would leave him for Rosalie. He might start to analyze all their past interactions, looking for signs that he missed. Eventually, he could become angry, not at Bella directly, but at the situation. He might also feel a sense of betrayal, not just from Bella but also from Rosalie in a way, since Rosalie is 'taking' Bella away from him.
One possible plot could be that Edward, perhaps influenced by some dark magic or a misunderstanding, decides he has more in common with Harry. Maybe he sees Harry's courage and selflessness and feels drawn to that world. He leaves Bella heartbroken, and Bella then has to find a way to move on while Edward starts a new life in the wizarding world with Harry as his new friend or more.
In some fanfictions, if Edward leaves Bella, she might become stronger and more independent. She could focus on her own life, perhaps pursue her studies or hobbies more intently. For example, she might finally decide to travel the world and discover new things on her own.
Maybe it's a group within the Ministry of Magic who oppose Harry's influence on Teddy. They might see Harry as trying to groom Teddy to be his successor in some way and want to stop them from leaving together to prevent this.
For Edward and Harry's relationship, it's likely to be broken and filled with hurt. Harry might feel betrayed and angry. As for Edward and Bella's relationship, it could start off with a lot of drama. People around them might not understand or accept it quickly.
One possible reason could be that Paul represents a different kind of life for Bella. Maybe he is more down - to - earth compared to the often brooding Edward. Paul might offer a simpler and more straightforward relationship without all the supernatural drama that comes with Edward.
Well, in some fanfictions, the author might justify it by showing that Edward has a deeper, more primal connection with Bella. Maybe he realizes that his love for Bella is something he can't ignore, and that his relationship with Harry was more of a passing acquaintance or a misjudgment in the first place.
Fem Harry might be heartbroken. She could initially try to win Edward back, using her magical abilities in creative ways. For example, she might create a magical memory of their time together to show Edward what they had was special.
Voldemort could still be the main antagonist. Even if Harry leaves his parents early, Voldemort might be hunting for him due to the prophecy. His dark presence would be a constant threat, driving Harry's actions and the plot of the fanfiction.
One possible plot twist could be that Edward realizes that Bella represents a more 'normal' life for him compared to Fem Harry. Maybe Fem Harry has some dangerous or complex magical powers that Edward fears, while Bella seems more human and easy to understand.