One possibility is that they leave to find a way to end the war secretly. They might journey to a far - off place to seek ancient magic or knowledge that could turn the tides without more bloodshed. Maybe they go to a hidden wizarding community that has been isolated from the conflict and holds the key to a peaceful resolution.
Perhaps they leave because they've lost faith in the current strategies. They could be on a quest to find a powerful magical artifact that was lost long ago. They believe this artifact has the power to either destroy the Dark Lord or at least weaken his forces so much that victory is assured. Along the way, they face various challenges and meet new allies.
One reason could be to find peace. After all the chaos and war in England, they might just want to get away from it all and start fresh somewhere else.
One possible plot could be that after the war, Harry and Hermione realize they have a deep bond that goes beyond friendship. They start to rebuild Hogwarts together and in the process, their feelings grow. They get married in a small ceremony at Hogwarts with their closest friends in attendance.
In some fanfictions, they might become closer friends than ever. After the war, they've shared so much trauma and victory that their bond deepens into a platonic relationship where they support each other through post - war rebuilding and dealing with their respective emotional scars.
One reason could be to search for a powerful magical artifact outside of Hogwarts that they believe is crucial for defeating a new dark threat. In fanfiction, they might feel that Hogwarts has become too dangerous or restricted in terms of resources for their mission.
One reason could be the media attention. After all they've been through, they might be tired of constantly being in the spotlight. The British wizarding press could be hounding them, so they leave to get some privacy.
Well, perhaps they go on a journey to discover new magical places that were previously unknown or forgotten during the war. Along the way, they face various challenges and meet new characters. Their relationship deepens as they rely on each other to overcome these obstacles. And they might also be on a mission to help those who were affected by the war in different ways.
Perhaps they fall in love. Gabrielle, having grown up and seen Harry's heroics in the war, might develop a deep admiration for him that turns into love. Harry, on the other hand, is drawn to Gabrielle's courage and her different perspective on magic, being from a different magical culture. They would face challenges though, like dealing with the post - war prejudices and the rebuilding of the wizarding world while trying to build their relationship.
Maybe he leaves because of a conflict with the Hogwarts staff. If the new headmaster or some teachers start implementing rules that Harry believes are wrong or unfair, like restrictions on defensive magic classes, he might decide to leave in protest.
If Ron and Hermione leave Harry in the fanfiction, it would change the entire fabric of the story. Harry would have to reevaluate his relationships with other characters. For example, he might turn to Neville for support more often. Also, the power dynamic within the wizarding world could shift. Since Ron and Hermione were important figures in the fight against Voldemort, their absence could give the Death Eaters an advantage, or it could inspire others to step up in unexpected ways. Harry would be forced to grow in ways he hadn't before, and it would be a journey full of self - discovery and new challenges.
One possible reason could be a misunderstanding. Maybe they misinterpreted something Hermione said or did, and in a fit of anger or confusion, they decided to leave. For example, if Hermione was working on a project alone without consulting them, they might have felt left out and left in a huff.