His Defense Against the Dark Arts skills would be very useful. He could use spells like Stupefy or Protego to defend himself against any threats he might encounter, whether it's from dark wizards or regular WW2 dangers. His knowledge of potions could also come in handy. He might be able to brew some basic potions to heal the wounded or create something to help him blend in or hide. And his flying skills on a broomstick could give him an advantage in scouting areas or making a quick escape if needed.
A great plot twist could be related to the time - travel itself. What if the way time - travel works in the WW2 era is different from how it is in Harry's time? This could lead to him getting stuck or being unable to use his magic properly. Or perhaps he discovers that a certain event in WW2 was actually caused by a wizard from the future (not just him) who was trying to correct something in the wizarding world's past. His attempts to fix things could then make the situation even more complicated as he unknowingly steps on the toes of a much larger time - traveling plan.
His father's friends, the Marauders, are very likely to interact with Harry if he goes back in time. He could become a part of their group, learn their secrets, and even participate in some of their pranks. Hagrid is another character. Harry has always had a special bond with Hagrid, and going back in time, he could strengthen that bond even more. And of course, Snape. Harry could try to change their relationship from the start and understand Snape better.
Definitely Hermione. Since she's Harry's closest friend and very intelligent, she would be deeply involved in any changes Harry tries to make. Also, Snape. Harry's actions in the past could completely change his relationship with Snape. And of course, Voldemort. If Harry goes back far enough, he could potentially stop Voldemort's transformation into the evil wizard he became.
Well, in this 'Harry Potter goes back in time' evil fanfiction, Harry might go back and change things to benefit Voldemort. Maybe he'd stop Dumbledore from finding out important information or interfere with the Order of the Phoenix's formation. He could also use time - travel to learn dark magic from the past and bring it to the present, causing destruction and fear among the wizarding community.
One interesting plot could be Harry going back in time to prevent the death of his parents. He might use his knowledge of future events to outwit Voldemort and his Death Eaters. Another plot could be Harry going back to his first year at Hogwarts and changing the way he interacted with certain characters like Snape, which could have a huge impact on the overall story. And he could also try to change the fate of some of the minor characters who died in the original story.
If Harry Potter goes back in time to teach fanfiction, it would be a really strange and interesting scenario. First of all, the wizarding world might not understand what fanfiction is at first. Harry would have to explain that it's a form of writing where fans create their own stories based on the original. He could inspire young witches and wizards to be more creative with the Harry Potter story itself. Maybe some of them would come up with really unique magical adventures that even J.K. Rowling hadn't thought of. Also, it could cause some chaos as the concept of fan - created stories might be a bit too modern for the old - time wizarding community.
Well, in some 'Harry Potter goes back in time' fanfics, Harry might go back to his first year at Hogwarts. He could then use his more advanced magical skills (from his future experiences) to solve the mystery of the Philosopher's Stone in a completely different way. This might involve him befriending different people earlier or even changing the course of how the challenges in the dungeon were set up.
Well, he might be able to change some of the bad things that happened. Maybe he could stop Umbridge from being so cruel. He could also be better prepared for the challenges that year brought, like the fight against Voldemort's forces.
Another possibility is that an evil wizard from the past, who was previously defeated, gets resurrected or empowered by Harry's time - traveling interference. This wizard could then become the main antagonist, using Harry's actions to their advantage and wreaking havoc on the wizarding world.