Sirius Black could be a trainer. Sirius, being Harry's godfather, would be highly motivated to train Harry. He could use the time - turner to go back to a time when he was at his peak in terms of combat skills and pass on his knowledge to Harry, especially in the areas of dark - wizard fighting and survival skills.
I think it could be Neville Longbottom. Neville has grown into a strong and confident wizard. He could have discovered boxing as a form of self - defense and then decided to teach Harry. Neville would bring his own unique perspective, having been bullied in the past. He'd train Harry to be both strong and fair in the boxing ring, emphasizing the importance of honor and respect in combat.
Maybe in the fanfiction, Harry uses the Time - Turner to train with long - dead great wizards. He travels back in time to learn from Merlin himself. Merlin could teach Harry ancient and powerful magic that has been lost over the centuries. This would give Harry an edge in his fight against Voldemort and also expand his understanding of the wizarding world's history.
There's a chance it could be a powerful house - elf. House - elves are known for their own brand of magic, and one particularly rebellious or enlightened house - elf could decide to train Harry in magic that is more about stealth and quick - thinking. After all, house - elves have to be very sneaky to avoid detection by their masters most of the time, and this kind of skill could be very useful for Harry in his battles against Voldemort.
It could be Dumbledore. Even if he's dead in the original story, in fanfiction, anything is possible. He might have left behind some kind of magic that allows him to train Harry from beyond the grave, or he could have faked his death to take Harry to a secret location for training.
Maybe it's a former Death Eater who has seen the error of his ways. He wants to atone for his past sins by training Harry to defeat Voldemort. He has inside knowledge of the dark side's tactics and uses this to make Harry a more formidable opponent.
If Harry Potter trains in the Time Chamber in fanfiction, he might discover new magical abilities that he didn't know he had. The Time Chamber is a place full of possibilities. He could use the extra time to study the history of magic more deeply, which could give him insights into how to defeat his enemies. He could also train his physical strength and endurance, which are often overlooked in magic training. This would make him a more well - rounded wizard, better equipped to face the challenges ahead.
It could be Hagrid. He has a lot of strength himself and might know some basic principles of building muscle. He could guide Harry on how to lift the heavy weights, using his own brute strength as an example.
Of course, there are often original characters in Harry Potter trains fanfiction. These could be new students who have their own secrets or powers. They might interact with the well - known characters, and sometimes they can even be the key to solving a mystery on the train, like uncovering a plot against Hogwarts that starts on the Hogwarts Express.
The consequences can be chaotic. It might lead to altered timelines. People could end up in the wrong era or at the wrong moment in history, changing the course of events in the wizarding world.
A Time - Turner fanfiction in the HG SS universe might involve Snape getting his hands on the Time - Turner. He could use it to go back and change some of his interactions with Lily, trying to prevent her from falling for James. This would create a whole new set of events and relationships, and the author could explore how the characters would adapt to these changes.