One possible storyline could be that Ron sacrifices himself to save Harry during a final battle against Voldemort. His death could be a turning point for Harry, making him more determined to defeat the Dark Lord. Another idea is that Ron is poisoned by a new dark magic spell created by Death Eaters, and his death causes Hermione to go on a vengeful quest against the Dark Arts. Maybe in a different take, Ron dies trying to protect his family from a group of rogue werewolves, and his death shatters the Weasley family but also unites them in a fight for justice.
Well, in some fanfiction where Ron dies, it could be related to a time - traveling accident. He might go back in time to change something crucial, but the magic goes wrong and he ends up sacrificing himself to fix the timeline. Or perhaps he gets caught in a magical experiment gone wrong in the Department of Mysteries. His death could lead to a rift between Harry and Hermione as they struggle to deal with the loss in different ways. Harry might become more withdrawn and focused on solitary vengeance, while Hermione tries to find a way to bring Ron back through some untested magic.
A plotline could be that Ron is killed in a magical accident during an experiment gone wrong. This accident might be related to some new, dangerous magic that he was trying to master. His death would shatter the trio. Harry would feel guilty because he might have been involved in the experiment in some way. Hermione would be grief - stricken and would pour herself into research to try and understand what went wrong. The fanfic could then explore how the wizarding community reacts to Ron's death, with some blaming Harry and Hermione, and others mourning the loss of a great wizard. It could also show how Harry and Hermione have to grow and become stronger individually to face the challenges that come their way after Ron's death.
One possible storyline could be that she sacrifices herself to save her family or friends. Maybe in a battle against dark wizards, she uses a powerful spell to hold off the enemy long enough for others to escape, but it drains her life force. Another could be that she succumbs to a mysterious and incurable magical illness that has been plaguing the wizarding world. This would lead to a very sad and poignant story as her loved ones struggle to come to terms with her impending death.
One possible storyline could be that in a battle against a powerful dark wizard, Harry sacrifices himself to save Hermione from a fatal curse. Another could be that he gives up his life to protect Hermione from a dangerous magical artifact that is about to explode. Or perhaps he takes on a deadly disease that was meant for Hermione, knowing it would cost him his life.
For Harry, it would be devastating. Ron has been his best friend since they were kids. He'd likely feel a deep sense of guilt, like he failed to protect him. Hermione would be heartbroken. Her relationship with Ron was complex and full of unspoken feelings. She might become more reclusive, burying herself in her studies to avoid the pain. The Weasley family would be shattered. Molly would be inconsolable, and the siblings would all feel a void. Ginny, in particular, would lose a brother who was always there for her.
A possible plot could be that the dragon is a new, unknown and extremely powerful species. Harry is sent to investigate it but underestimates its power and gets killed.
One possible theme could be betrayal. If Harry hates Ron, there might be a backstory of Ron doing something that made Harry feel deeply betrayed. Another theme could be jealousy. Maybe Ron has something that Harry desires, which leads to this hatred.
Another reason could be a misunderstanding. Maybe he overhears something Harry and Hermione say and misinterprets it as them not wanting him around anymore. So, in a fit of anger and hurt, he leaves. This can lead to a lot of angst and drama in the fanfiction as the other characters then have to figure out how to get him back.
A plotline could be that Ron betrays Harry's trust by sharing a secret with someone he shouldn't. Harry is initially furious but then realizes Ron was manipulated. Harry forgives Ron, and they work together to fix the mess caused by the shared secret. For example, if the secret was about a new spell they were developing, they have to stop the wrong people from using it.
Perhaps Ginny discovers some hidden secrets or powers within herself as she copes with Harry's death and the pregnancy. Maybe she has to deal with people who question her ability to raise the child without Harry. The story could also focus on how the child grows up with the legacy of Harry, and whether the child inherits any of Harry's special abilities or traits.
One possible storyline could be that Voldemort manages to attack the Dursleys' house before Hagrid can arrive to take Harry to Hogwarts. Maybe a Death Eater discovers Harry's location early on and Voldemort decides to end the threat immediately. Another could be that some dark magic from Harry's parents' past catches up to him as a baby, and he succumbs to it, never making it to Hogwarts. It could also be that a wizarding accident occurs in the Muggle world that involves Harry and results in his death, like a misfired spell from a rogue wizard in hiding.