They might run away because they want to explore the wizarding world on their own terms. Without the restrictions of Hogwarts or the watchful eyes of the Ministry of Magic.
Well, perhaps they run away to escape the chaos at Hogwarts. Maybe they've discovered a dark secret within the school that puts their lives in danger. So they decide to flee and find a hidden place in the Forbidden Forest to stay safe.
One possible plot could be that they run away to escape the dark forces that are taking over the wizarding world. They might be on a quest to find a powerful magical artifact that could turn the tide of the battle. Along the way, they face various challenges like dangerous creatures and spells.
Voldemort and his Death Eaters would be obvious antagonists. They could be constantly hunting for Harry and Tonks as they are seen as threats to Voldemort's plans for total domination of the wizarding world.
One possible plot could be that Harry and Daphne run away to escape the chaos of the wizarding world. Maybe they are tired of the constant danger and politics. They find a secluded place, perhaps a hidden cottage in the Forbidden Forest. There, they learn to live off the land, grow their own food, and discover new magic away from the prying eyes of others.
Harry, the boy who survived Voldemort's attack as a baby and became a key figure in the fight against the Dark Lord. And Daphne, a character who may not be as prominent in the original books but takes on a major role in this fanfiction. Their personalities and how they interact with each other while on the run are what make up much of the story.
The remaining Death Eaters could be the antagonists. Even though Voldemort is gone, there could be some who still hold a grudge against Harry and want to finish what their master started. They might track Harry and Ginny down as they run away, creating dangerous situations for the couple.
One possible plot could be that they run away to escape some dark magic threat that Voldemort's remaining followers are planning. They might go into hiding in a remote wizarding village, where they have to learn to fend for themselves without the help of the Order of the Phoenix. Along the way, they could discover new magical abilities or ancient spells that help them in their fight against the dark forces.
Maybe some jealous students from Hogwarts or Beauxbatons. They don't like the idea of Harry and Fleur being together and running away to have an adventure. So they try to sabotage their plans, perhaps by spreading false rumors or even using minor hexes to slow them down on their journey.