One possible plot could be that the puppy is a key to a hidden treasure. Harry has to follow the puppy's instincts to find the treasure which could be something very important for the wizarding world. Another plot could be that the puppy is in danger from a dark wizard who wants to use it for some evil experiment, so Harry has to protect it at all costs.
The puppy could be a symbol of hope in a dark time for the wizarding world. Harry finds the puppy during a difficult period, and as he nurtures it, it starts to bring positive changes to the people around him. People are drawn to the puppy's innocence and it gives them the strength to fight against the encroaching darkness. This plot could explore themes of hope, love, and the power of small things to make a big difference.
A plot might involve the puppy being a magical hybrid. Harry discovers that the puppy has the powers of both a normal dog and a magical creature. As he tries to understand these powers, he gets into all sorts of adventures. Maybe the puppy can communicate with other magical animals, and Harry uses this ability to solve a mystery in the Forbidden Forest.
Well, one possible plot could be that Harry's friends race against time to find an antidote. Maybe Hermione uses her vast knowledge of magic to research ancient spells and potions that could counteract the poison. Another could be that the poison has some strange side - effects on Harry, like giving him temporary new magical abilities while also slowly killing him, and he has to figure out how to use these new powers to save himself. Or perhaps the poisoning is part of a bigger conspiracy within the wizarding world, and Harry's poisoning is just the first step in a plot to take over Hogwarts or the Ministry of Magic.
One possible plot could be that in an alternate universe, Harry and Ron got married for some reason, perhaps due to a magical bond gone wrong. Then as they grow older, they realize they have different life goals and values, leading to their divorce. For example, Harry might be more focused on fighting dark magic still while Ron wants a more peaceful domestic life.
One possible plot could be that Harry is magically compelled to submit. Voldemort might use some dark magic or a powerful artifact to control Harry's mind. Another plot could be that Harry submits out of a false belief that it will save his friends. For example, Voldemort could deceive Harry into thinking that if he surrenders, his friends will be spared. Or perhaps Harry is so worn down by the fight that he gives in, thinking there's no other way, and then the story could explore how Voldemort uses Harry's submission to his advantage, like making Harry a tool to gain the loyalty of other wizards.
Perhaps there's a fanfiction where Ron is forced to work at a dentist's office as a punishment. He has to deal with all kinds of strange patients, from veelas with very sharp teeth to giants with massive molars. His misadventures in the dentist office, trying to use magic to get out of his work or accidentally making things worse, could make for a really funny and engaging plot.
Well, it's a rather dark concept. One possible plot could be that in a dark, alternate universe, Harry is under some sort of dark magic influence which makes him act against Hermione's will. But this is very much against the essence of their characters in the original series.
Well, one possible plot could be a wizard who has picked up the Muggle habit of smoking cigarettes and how it affects his magic. Maybe his spells start to go awry because the smoke interferes with the magical energy. Another could be a group of Hogwarts students who discover cigarettes in the Forbidden Forest and have to deal with the consequences of this Muggle item in a magical place. But these are just very speculative ideas as it's not a common type of fanfiction.
One possible plot is that the puppy was abandoned in the basement by its previous owner. It had to struggle to survive with little food and water until someone discovered it.
One possible plot could be that Harry finally snaps after years of abuse from the Dursleys. He might discover some dark magic that he uses against them in a fit of rage. Maybe he finds out they were involved in something really evil that endangered his life all along, and that's what drives him to murder them.
One possible plot could be that they disappear to a parallel magical dimension. In this dimension, they have to learn new forms of magic to survive and find their way back home. Maybe they encounter strange magical creatures that are unknown in their own world.
One possible plot could be that Fleur betrays Harry in a Triwizard Tournament - related task, perhaps by secretly helping the other champions. Another might be that Harry discovers Fleur has been passing information about the Order of the Phoenix to the wrong side. Maybe Fleur betrays Harry's trust regarding a personal secret, like his feelings for Ginny, and uses it to her advantage in some way.