One possible storyline could be that the fairy dog is a gift from Dumbledore. It has the ability to sense dark magic and helps Harry detect threats before they become dangerous. For example, it could sniff out a hidden Death Eater's spell.
Common themes often include magic and mystery. The fairy dog usually has some special magical powers that are a mystery to be uncovered. Also, the theme of loyalty is present as the dog is loyal to Harry. In these fanfictions, the dog might also represent a new form of magic in the wizarding world that Harry has to learn about and adapt to in his fight against the dark forces.
One possible storyline could be that Harry's experience with cancer makes him more resilient. He might have faced his own mortality at a young age, so when he enters the wizarding world, he has a different perspective on danger. For example, he may not be as afraid of Voldemort as others because he has already fought a life - threatening battle with cancer.
Another idea is that there is a prophecy that requires Harry's early death to save the wizarding world in a different way. He discovers this prophecy and, being the selfless hero he is, willingly accepts his fate. His death then triggers a series of events that lead to a new era of peace and harmony in the wizarding community, but his friends and allies still deeply mourn his loss.
A possible storyline could be that Harry's blue hair is a sign of a new and powerful magic within him. This new magic could attract the attention of dark wizards who want to either control or destroy him. So, he has to learn to control this new aspect of himself while also dealing with the normal Hogwarts drama.
Finding a fanfiction specifically about Harry Potter having a fairy dog can be a bit of a challenge. Fanfiction is a very diverse world, and while there are countless stories about Harry Potter, the ones with this particular element might be scarce. However, you could try to write your own. Imagine how the fairy dog could interact with Harry's magical world. It could have special powers that help Harry in his adventures at Hogwarts. You could even create a whole new story arc around this fairy dog and Harry's relationship with it.
One possible storyline could be that the service dog helps Harry deal with his PTSD from all the traumas he faced, like the death of his parents and the battles at Hogwarts. The dog could sense when he's about to have a panic attack and comfort him.
One possible storyline could be the children of the original characters starting their own adventures at Hogwarts. They might face new threats that are related to the past but in a different way. For example, a new dark wizard could be trying to revive the ideas of Voldemort and the kids have to stop it using the knowledge passed down from their parents.
One possible storyline could be that the Dursleys' pet, perhaps their cat, discovers Harry's wizarding abilities before the Dursleys do. It might then have some comical or tense interactions with Harry, like trying to stop him from using magic in the house for fear of getting in trouble itself.
One possible storyline could be that Harry is on the run from dark forces again and the tent serves as his only safe haven. He might be using magic to conceal the tent and make it a cozy, self - sufficient living space, with magical wards all around it for protection.
Perhaps in the fanfiction, Harry discovers that his grandparents had a hidden magical legacy. They had been working on a powerful charm to protect the Potter family long before Harry was born. Harry finds clues about this charm and with the help of his friends, he tries to complete it, believing it could be the key to finally defeating Voldemort.