A possible plotline could be that they are on the run from the Ministry of Magic. Sirius knows some secrets about Harry's past that make it dangerous for them to stay in England. So, they head to a far - off magical land where they try to start a new life in hiding.
In the fanfic, Sirius and Harry leaving England could lead them to a hidden magical island. There, Harry could learn different kinds of magic that are not taught at Hogwarts. Sirius would be able to protect Harry better as they are far away from the threats in England. They might also start a new life where Harry can grow up without the constant fear of Voldemort's followers finding him.
It's also likely that they go to Romania. Given Sirius' knowledge of the wizarding world, he might know of a safe haven there. In Romania, they could be around dragons and learn about dragon - related magic. Harry would surely be fascinated by the dragons and it could be a place where he can grow stronger in a different kind of magical environment.
One possible plotline could be Ginny leaving Harry due to the pressure of his fame. She might feel overshadowed and decides to pursue her own life away from the spotlight. Another could be a misunderstanding. Maybe she misinterprets Harry's actions or words and in a fit of anger, leaves him. Or perhaps an external threat forces her to leave to protect Harry, like a dark wizard threatening her to stay away from him so as not to be used against Harry.
A third plotline could be that Harry's choice of electives attracts the attention of a new and mysterious teacher. This teacher has their own agenda and uses Harry's interest in the electives to manipulate him into doing things that may or may not be in his best interest. It could lead to a lot of self - discovery for Harry as he tries to figure out the teacher's true motives while also enjoying his electives.
Well, if Sirius takes Harry to Voldemort in a fanfic, it would be a huge deviation from the normal story. One possibility is that Sirius is trying to make some sort of deal with Voldemort to save others. But Voldemort is not one to be trusted. Harry, being the Boy Who Lived, is a valuable asset to Voldemort for his own dark purposes. It could lead to a tense stand - off where Harry has to use his own wits and magic, even at such a young age, to try and escape the clutches of the Dark Lord.
In this fanfic, Sirius could be tipped off that the Dursleys are planning to send Harry away to some even worse place. So he swoops in. They flee to a remote wizarding village. There, Harry meets other wizards and witches who are sympathetic to his cause. Sirius helps Harry train in combat magic, as he anticipates future battles. Harry also gets to explore his magical heritage more freely without the Dursleys' restrictions.
One possible reason could be that Sirius, being Harry's godfather, feels it's his right and duty to protect Harry. He might not trust Hagrid fully to keep Harry safe, especially considering the dangerous situation Harry is in. Sirius has a strong connection to Harry and wants to be directly involved in his upbringing and protection.
One possible plotline could be Sirius taking on a more parental or guiding role in Harry's life in a non - traditional way due to the 'age play' aspect. For example, he might help Harry learn magic in a more hands - on and protective manner.
One possible reason could be to protect Teddy. Maybe there are some threats in England that Harry wants to keep Teddy away from. For example, there could be some dark wizards or dangerous magical creatures that target Teddy for some reason.
A plotline might involve Sirius and Harry living together and Sirius trying to give Harry a normal family life. He could cook for Harry, help him with his homework, and they could have a pet together. Maybe they get a Kneazle, and Harry learns to take care of it with Sirius' guidance. This would be a contrast to the cold and often neglectful treatment he received from the Dursleys. And as Harry grows, Sirius could introduce him to his old friends like Remus Lupin in a more family - like setting, strengthening Harry's support system.