His control over spacetime would likely give him an edge in battles. He could freeze time to strategize or move his allies to safer positions. In terms of the plot, it would make him a much more dominant force, changing the power dynamics among the characters. Maybe he could even travel to different dimensions within the wizarding world and discover new magic, which would then introduce whole new sub - plots.
If Harry Potter controls spacetime, it would completely disrupt the normal flow of the story. He could skip over parts of the plot that were originally meant to be challenges, like the Triwizard Tournament. He might also be able to visit the future and learn about upcoming threats, which would then change how he prepares for them in the present, thus greatly affecting the overall plot development.
Well, it could change the whole sequence of events. For example, he might be able to go back in time to prevent certain tragedies from happening in the wizarding world. This would rewrite a lot of the original story's plotlines as we know them.
He might have enhanced magical perception. Since he can control spacetime, he could be more in tune with the magical energy that flows through different times and spaces. This would allow him to sense things others can't.
Well, in some fanfictions, Harry might gain control over the Wizengamot through his heroic deeds. For example, if he saves the wizarding world from a great threat again, the public support could be so strong that he can influence the Wizengamot's decisions. He could also use his knowledge of magic, especially some ancient or powerful magic that gives him an edge in the political arena of the wizarding world.
In some fanfictions, it could be through intense magical training and study. Maybe he discovers an ancient spell or a hidden magical technique that allows him to assert control over the dangerous Fiendfire.
Well, it might be related to his connection with the Dark Lord. Since Voldemort was able to control the basilisk before, and Harry has a unique connection to Voldemort through the scar. This connection could be manipulated in some way in the fanfiction, allowing Harry to take over the control of the basilisk from Voldemort.
Well, it could change the whole dynamic. He might use it to manipulate enemies, like making Voldemort's followers turn on him. This would speed up the end of the war perhaps.
In some fanfictions, Harry might gain the ability to control Fiendfyre through intense magical training. He could be mentored by a powerful wizard who teaches him the dark arts in a controlled way, allowing him to master Fiendfyre. Another possibility is that he inherits some ancient magic that gives him this power. For example, if he discovers a long - lost family heirloom that has the magic to control such dangerous spells.
The protective brother can change the plot significantly. For instance, if he's highly skilled in magic, he can prevent Harry from getting into some of the dangerous situations he faces in the original series. He might be able to stop Harry from entering the Triwizard Tournament by interfering with the Goblet of Fire's magic. This would then change the entire course of that year at Hogwarts and Harry's relationship with Cedric Diggory and others.
Well, at first, he might be a bit scared and unsure of his new ability to control the weather. But as he starts to experiment, he gains confidence.
Maybe he discovers it during a particularly intense storm at Hogwarts. He could be in a dangerous situation, like being chased by some dark creatures, and in a moment of desperation, he unknowingly summons lightning to protect himself.
Well, it could change the relationships in the story. For example, he might use it to make enemies become allies, like turning some Death Eaters to his side. This would completely shift the power balance in the fight against Voldemort.