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 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.
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.
One popular fanfiction could be 'The Dark Triumphant' where Voldemort wins in a very different way. It shows how Draco, Hermione and Ron are caught in a world ruled by Voldemort's new order. Another one might be 'A World Turned' which explores how the relationships between Draco, Hermione and Ron change when Voldemort is victorious. There are also some that focus on the moral and ethical dilemmas the characters face in such a dark world.
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 'loki wins the war fanfiction', a frequent theme is Loki's self - discovery. As he battles through the war, he learns more about his own capabilities and his place in the universe. Also, the theme of subverting expectations is common. Everyone expects Loki to lose or cause chaos in a negative way, but in these fics, he uses chaos to his advantage and wins. Another theme could be his complex relationships with other characters being reforged through his victory.
One interesting aspect could be the exploration of Voldemort's character in a way that he actually wins. It might show his different strategies and how he would rule the wizarding world. For example, it could delve into how he would restructure Hogwarts or deal with the remaining resistance.
In many such fanfictions, Draco often has a complex reaction. Since his family was on Voldemort's side initially, he might initially be in a position of power but then start to question Voldemort's methods. Hermione, being the intelligent and strong - willed character, usually tries to find ways to resist. Ron, who is loyal to his friends, stands by Hermione and they often try to form some sort of resistance together.
The obvious key supporters would be the Death Eaters. They were always loyal to Voldemort. Bellatrix Lestrange would be one of the most prominent ones, being extremely fanatical in her devotion to him.