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.
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 wizarding world would likely be plunged into a state of despair. Harry was seen as the 'Chosen One' to defeat Voldemort, so without him, there would be a sense of hopelessness. Voldemort would probably have free rein to impose his dark rule without much opposition at first.
When Fem Harry joins Voldemort, it completely flips the power balance. The Ministry of Magic, which was already in a tenuous position, would be even more unstable. They would be unsure how to deal with a situation where the once - hero is now on the side of the dark lord. The Hogwarts students would be divided. Some might be inspired by Fem Harry's actions, thinking there's more to the story, while others would be heartbroken. This would also lead to a change in the way magic is perceived. Dark magic might start to seem more appealing to some, and light magic less so, creating a moral gray area in the wizarding world.
The wizarding world would be in chaos. Harry, being Voldemort's right - hand man, would be a huge threat. He knows many of the Order of the Phoenix's secrets. Hogwarts would no longer be a safe haven. The students would be in constant fear as Harry could use his inside knowledge to help Voldemort attack the school. The Ministry of Magic would also be in a difficult position as they had once relied on Harry to be their savior.
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.
There are quite a few. One popular one involves Voldemort using a powerful dark curse that the Order of the Phoenix couldn't counter.
Well, the pure - blood families who were on the fence about Harry might finally accept him wholeheartedly. They would see that he was the one who could truly rid the wizarding world of Voldemort. However, there would also be those who would be critical. Some of the more traditional wizards and witches might think that using a shooting method was too violent and un - wizard - like. They might worry that this sets a bad precedent for future conflicts in the wizarding world. But overall, the general public would be relieved that Voldemort was finally gone and would likely celebrate Harry as a savior.
The wizarding world would be in shock. The two most polarizing figures being mates would shatter the existing power dynamics. The Death Eaters might split, some following Voldemort's new path with Harry, while others would rebel. The Order of the Phoenix would also be divided, with some believing in the power of love and this strange bond, and others seeing it as a trick by Voldemort.
There would be a great deal of shock and confusion. Many would start to question everything they had believed in. Families would be divided. Some might think that Harry had been corrupted by Voldemort's dark magic and try to find a way to reverse it. The Ministry of Magic would be in a panic. They would try to come up with new strategies to deal with this new threat. Teachers at Hogwarts would be worried about the safety of their students. And the Death Eaters, while initially happy to have Harry on their side, would also be suspicious of his motives and loyalty.
In fanfiction, Harry Potter can change the Wizarding World in many ways. For example, he might introduce new spells or magical concepts that were not in the original series. He could also have different relationships with other characters, which in turn would affect the social dynamics of the Wizarding World.