One possible plotline could be a world under totalitarian rule. Voldemort would likely impose strict pure - blood supremacy laws. Muggle - borns would be persecuted and forced into hiding or slavery. Hogwarts might be turned into a place that only teaches the Dark Arts and Voldemort's ideology. Another plotline could involve the resistance. Some brave witches and wizards might form an underground movement to fight against Voldemort's regime, with their own struggles and sacrifices.
If Voldemort won the war in fanfiction, the wizarding world would experience a total transformation. First, the Ministry of Magic would be under his complete control. His Death Eaters would be in charge of all the important positions. The concept of justice would be warped, and any form of opposition would be brutally suppressed. The wizarding economy would likely suffer too as Voldemort's focus would be on power and domination rather than development. Also, the relationships between different magical creatures would be disrupted as Voldemort has no respect for any form of life other than his own pure - blood ideology.
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.
If Voldemort won the first war in fanfiction, the social structure of the wizarding world would be turned upside down. Death Eaters would be in power everywhere. The use of Dark Magic would be widespread. The very fabric of the wizarding society that values love, friendship and equality would be torn apart. Wizards would live in constant fear, and any form of resistance would be brutally crushed.
Well, if Voldemort emerged victorious, it's likely that pure-blood supremacy would rule. Muggles and Muggle-borns would face severe persecution, and the very fabric of the magical society as we know it would change drastically.
In many WBWL fanfictions where Harry is powerful, he often uses his hidden powers to resist Voldemort's regime. For example, he might have an extraordinary control over magic that was suppressed in the original story. He could use this power to secretly help the resistance, like teaching other wizards and witches how to fight against the dark arts in new and unexpected ways.
One great 'Cold Harry Potter WBWL' fanfiction is 'The Boy Who Lived Again'. It shows a darker and more complex wizarding world under Voldemort's rule. Harry has to hide his true identity and find a way to fight back.
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.