Well, perhaps in this fanfiction, Voldemort wins the first war because he discovers a way to nullify the power of love, which is the greatest weapon against him. He could have found a dark spell or an ancient potion. With this advantage, he's able to kill Harry Potter's parents without hesitation. Then, without the sacrifice of love to protect Harry, he kills Harry as well. This demoralizes the wizarding world completely, and with no hope left, they are easily conquered by Voldemort.
Maybe Voldemort manages to defeat all the major resistance. He could start by taking out key members of the Order of Phoenix one by one. For example, he might ambush Sirius Black early on, weakening the Order's intelligence network. Then, he'd target Dumbledore, perhaps using some dark magic that even Dumbledore couldn't counter. With the two biggest threats out of the way, he'd be able to overrun the wizarding world, starting with Hogwarts. He'd install his own regime, making pure - blood supremacy the law and punishing those who oppose him severely.
Bellatrix Lestrange would probably be a main character. As one of Voldemort's most loyal followers, she'd be at the forefront of his new regime. She might be in charge of torturing those who oppose Voldemort.
In the 'kronos wins the war fanfiction', it might be that Kronos discovers a powerful weapon or magic. This newfound power allows him to overpower his enemies one by one. He could also be very cunning in manipulating the internal conflicts of his opponents, which in the end helps him win the war.
In some fanfictions, Voldemort wins through a change in the prophecy's interpretation. If the prophecy was misread, it could give him an edge he didn't originally have.
Perhaps Harry joins Voldemort because he believes Voldemort's lies about the wizarding world. Voldemort could tell Harry that the Order of the Phoenix is actually trying to use him and that he would be better off with him. In this scenario, Hogwarts would become a very different place. Teachers would be in a state of panic, not knowing how to deal with Harry's new allegiance. The other students would be terrified, and some might even be swayed to Voldemort's side as well, seeing Harry as a role model. It would be a chaotic and dark situation.
In voldemort wins fanfiction, there might be a lot of character development that goes against the original. For instance, some of the Death Eaters could become more complex characters. They may start to question Voldemort's extreme rule once they see the destruction it causes. Also, there could be a different take on the prophecy and how it was misinterpreted which led to Voldemort's win.
Well, if Robb Stark wins the war in the fanfiction, he would likely have a major impact on the political landscape. He could reshape the power dynamics, perhaps even expanding his influence beyond the North. He might also be in a position to avenge his father's death more thoroughly and bring stability to the realm. Additionally, his victory could lead to different character developments for those around him, like his mother or his bannermen.
Perhaps in this fanfiction, Naruto discovers some ancient magic similar to Voldemort's dark magic. He becomes intrigued by Voldemort's rise to power and starts to study his methods. However, Naruto's friends would be horrified as they see him drifting towards the dark side. Naruto might think he can use Voldemort - like tactics for good, but it's a slippery slope.
In an 'Ash Wins Kalos League' fanfiction, one possible plot could be that Ash finally masters his Mega Evolution techniques better than ever. He has a series of intense battles where he outsmarts his opponents. His bond with his Pokémon, especially Greninja, reaches new heights. They use their unique moves in creative ways to defeat strong adversaries and claim the victory in the Kalos League.
Well, perhaps in this fanfiction, Ash has trained his Pokémon to their peak performance in Unova. He might have developed new strategies specifically for the battles in the Unova League. His Pokémon could have unique evolutions or new moves that give him an edge. For example, his Pikachu could have learned a super - powerful electric move that surprises his opponents.
Well, it could be that Ash has trained with some rare and powerful Pokémon in the Kalos region that are not typically part of his main team. This new addition to his arsenal helps him win the league. Also, he could have received some secret training from a legendary Pokémon trainer in Kalos, which gives him the upper hand in all his battles.