In a 'Harry Fights Alone' fanfiction, the plot could involve Harry being betrayed by his friends. Maybe they are put under a spell that makes them turn against him. Harry then has to face the dark forces all by himself. He might have to rely on his own wits and the few spells he has mastered alone. For example, he could be in a situation where he has to battle Voldemort's minions without Ron and Hermione to back him up, and he discovers new powers within himself during this struggle.
Perhaps Harry is separated from the wizarding world for some reason. He could be in a different dimension or trapped in an ancient, forgotten place. Here, he has no one but himself to fight the strange and dangerous creatures that lurk. He might use his knowledge of magic in innovative ways, like combining spells he learned at Hogwarts in unexpected ways to fend off the threats. This isolation could also lead to his personal growth as he learns to trust his instincts more.
In a 'naruto fights alone fanfiction', it could be that Naruto is on a mission in a foreign land where he has no support. The local ninjas are all against him for some reason. He has to use his Nine - Tails chakra in a more discreet way as he can't afford to go all - out and draw too much attention. He might befriend some local animals or spirits who help him in a more indirect way, like giving him information about the enemies' weaknesses.
It could be a new dark wizard who is more powerful and cunning than Voldemort. This wizard might have been in hiding all this time, biding his time to take over the wizarding world. With Harry being alone, he would pose a great threat as Harry doesn't have his friends to support him in this fight.
Perhaps in this fanfiction, the court in the wizarding world has been corrupted. Harry discovers some dark secrets within the court system. He then decides to fight against it to bring justice. He might start by gathering evidence against those who are misusing their power in the court.
Well, perhaps Teddy is in some sort of danger. Maybe he's being targeted by dark wizards because of his connection to Harry. Harry, being the heroic type, decides to fight to protect Teddy. He could use his magic skills and the support of his friends like Hermione and Ron to keep Teddy safe.
Well, it could be that Harry is facing some new dark forces in the fanfiction. Maybe he discovers a hidden group of dark wizards who are plotting against Hogwarts, and he decides to fight back to protect his school and friends.
The fanfiction could start with Harry being magically transported to Kaguya's world. He'd be confused at first but quickly realize he has to fight to survive. Harry's wand would be his main weapon. He might learn some new spells during the fight, perhaps a spell that can nullify Kaguya's ability to freeze time. As the fight progresses, both characters would start to respect each other's power, leading to an unexpected alliance at the end.
In a Harry Potter vs Odin fanfiction, it could start with Odin thinking Harry is a threat to the Asgardian way. Harry, on the other hand, is just trying to protect Hogwarts. The fight might involve Odin using his powerful spear Gungnir while Harry combines his Patronus charm with some of the more powerful dark arts spells he's learned in his fight against Voldemort. But Harry would be reluctant to fully embrace the dark arts as it goes against his nature.
Well, perhaps Harry has to use his magic skills to resist some sort of spell or curse that is trying to force him into hibernation. Maybe there are dark forces at work trying to put him out of action for a long time.
Maybe Harry Potter discovers a new dark threat in the fanfiction and has to fight it using his magical skills and the support of his friends. He could be facing a powerful dark wizard who has found a way to break through Hogwarts' defenses.
In this fanfiction, perhaps Harry could use his wits more than magic. He might outsmart his enemies by setting up elaborate traps, like he did with the troll in the first year but on a much larger scale. He could use his knowledge of Hogwarts' secret passages to his advantage, leading his foes into dead - ends.