One possible plot could be that Harry is sent back in time by some powerful magic or a time - turning device. Once in the past, he realizes that he has a mission to become a fighter for a cause that was lost in the future. He might start by training himself in the older, perhaps more dangerous forms of magic that were forgotten in his time. He then begins to form alliances with wizards and witches from the past who can help him in his fight. This could involve battles against dark wizards other than Voldemort, or even against the Ministry of Magic if they were corrupt at that time. The story could also explore how his actions in the past start to reshape the future he came from in unexpected ways.
There could be many 'Harry Potter is a fighter back in time' fanfics out there. You could also join some Harry Potter fan communities on social media platforms like Reddit or Facebook. The members there are often very knowledgeable about different fanfics and might be able to point you in the right direction. Another option is to use Google and type in the exact title or a close variation of it, along with 'Harry Potter fanfic' to see if it shows up.
In a 'Harry Potter is a fighterback in time' fanfic, the plot could involve Harry going back in time for various reasons. It could be to save someone who died in the original timeline, like Sirius Black. He would then have to face the challenges of the past while also trying not to disrupt the future too much. He might also have to deal with the younger versions of his friends and teachers, and how they react to his knowledge from the future. And of course, he would be fighting against the threats of that time period, which could include Death Eaters or other dark wizards. He would need to use his skills and magic in new ways, adapting to the different time and its rules.
Well, it could be that Harry Potter discovers he has a connection to Sparda. Maybe he has some latent power that is related to Sparda's abilities. He then has to learn to control this new power while still dealing with the threats in his own wizarding world. And perhaps he uses his new - found powers to fight off dark wizards in a more badass way.
One possible plot could be that a new, extremely powerful and irresistible poison is created by a dark wizard. The main characters, like Harry, Hermione, and Ron, have to find an antidote before it spreads and kills many students at Hogwarts.
Perhaps it's a fanfic that takes the concept of Persephone's abduction in Greek mythology and applies it to the Harry Potter universe. The 'his' could be a dark wizard who kidnaps Persephone, a powerful witch, and the plot could be about her friends in Hogwarts trying to rescue her. Or it could be a more romantic take, where 'his' is a hero - type character and Persephone is in some sort of danger related to the ongoing battle between good and evil in the wizarding world, and he has to save her.
The plot could be about Harry Potter taking a nightly stroll in the Forbidden Forest. He might be feeling restless and decides to explore on his own. During the stroll, he stumbles upon a rare magical creature that he's only read about in books. This encounter could lead to a new adventure as he tries to protect the creature from other threats in the forest.
In a possible plot, the Ministry of Magic decides to introduce a new form of combat training based on Street Fighter to better prepare wizards for dark threats. Harry Potter is skeptical at first but then realizes the potential. As he trains, he uncovers a conspiracy within the Ministry that is using this new training method for their own evil purposes. He then has to use his skills from both magic and Street Fighter to expose the conspiracy and save the wizarding world.
Maybe Harry travels back in time and realizes he is Godric in a past life. He then uses his knowledge from the future to change some events in Godric's time, which in turn has a big impact on the future of the wizarding world he came from.
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.
Maybe it would involve Harry and his friends having to fight off hordes of zombies in Hogwarts. The Forbidden Forest could be full of zombie creatures.