Yes, it could be. It's not a typical concept we usually hear about. Fanfiction usually focuses on new adventures or relationships for Harry Potter, but the idea of him 'fighting' fanfiction is quite novel.
It really depends on the situation. If it's in a magical environment like Hogwarts, Harry might have an advantage with all the spells he knows. But if it's an open area, Ben with his Omnitrix could transform into an alien like Way Big and overpower Harry with his size and strength.
Well, it could be that the authors of fanfiction want to explore different aspects of the Harry Potter universe other than fighting. Maybe they're more interested in the relationships between characters or the daily life at Hogwarts.
In some fanfictions, Harry Potter might learn to fight through intense training with experienced wizards or by discovering hidden magical combat techniques.
One possible reason could be the Ministry's corruption. In the original series, the Ministry often showed signs of being influenced by dark forces or being overly bureaucratic and not seeing the real threats. So in the fanfiction, this might be a driving factor for Harry to oppose them.
Often, Harry's knowledge of fighting in fanfiction comes from intense self-study and practice, as he realizes the importance of being well-prepared for the challenges ahead.
It could be a deranged fan who has found a way to bring their dark fanfiction ideas to life. This person might be jealous of the original story and wants to create chaos in the wizarding world through their fictional concepts made real.
One possibility is that Harry discovers a world where fanfiction has the power to change the real wizarding world. So he has to train to fight against the alterations caused by these fictional stories. For example, if a fanfiction made Hogwarts a place full of dangerous magical beasts that don't belong there, Harry would need to train to restore order.
Well, it could be about Harry Potter getting into a battle to gain custody of something or someone. Maybe it's about him fighting for the custody of a magical creature or even a young wizard in need of protection. He might use his magic skills and the support of his friends to win the custody battle in this fan - made story.
He would bring a whole new dimension of power. His magic could be used for offense, like powerful curses, and defense, such as protective enchantments. The Avengers might be initially skeptical, but once they see what he can do, they'd welcome him. For instance, he could create a magical shield around the team using a spell like 'Protego Maxima' which could withstand Thanos' attacks better than their normal shields.