If Voldemort killed Harry Potter, then the entire wizarding world would face great turmoil and change. The following is a possible plot development:
The death of Harry Potter will make Lord Voldemort the most powerful existence in the Wizarding world. He may try to take revenge or rule the entire Wizarding world. Other wizards and wizards might begin to feel uneasy and afraid and seek power against Voldemort.
The death of Harry Potter could cause a great deal of chaos and panic, and the Wizarding world could descend into chaos and panic. The government and the army may begin to act to protect the Wizarding world from further threats.
The death of Harry Potter may lead to political unrest, and some wizards and wizards may begin to organize their own forces to try to fight against Voldemort and his followers.
Voldemort may try to use Harry Potter's death to gain more power and influence. He may regain control of the Wizarding world through other means.
The death of Harry Potter may spark a profound discussion about magic and morality. Other wizards and wizards might begin to think about their views on magic and morality, and how they should face Voldemort and his followers.
If Voldemort kills Harry Potter, the Wizarding world will face great upheaval and change, and will need the efforts and cooperation of other wizards and wizards to meet these challenges.
Well, it could have led to a total shift in the magical world's balance. Voldemort might have reigned supreme, and the fight against evil would have become even more desperate.
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.
Another storyline might involve a betrayal within the wizarding world. Suppose someone close to Harry, perhaps a fellow student who has been secretly turned to the dark side, sets a trap for Harry. Voldemort then takes advantage of this situation and kills Harry, which would have a huge impact on the entire wizarding community as they lose their hope symbol.
Maybe it's a completely new character created for the fanfiction. This new character could be from a far - off land, with a different kind of magic that Voldemort is interested in. Their marriage could be a way for Voldemort to gain access to that new magic or territory.
Another option could be a Muggle - born witch. Voldemort might have been intrigued by her in a different way compared to his hatred for most Muggle - borns. But as his views on pure - blood supremacy become more extreme, she can't bear to be with him any longer, and their relationship ends.
It could be Dumbledore. He was always one to oppose Voldemort and a howler from him would be a very interesting plot point. He might use it to taunt Voldemort or to try and make him second - guess his actions.
Harry Potter would likely be one of the main characters. Since Ginny is his love interest in the original series, he would surely be affected by her marriage to Voldemort. He might be the one to try and rescue her or stop the marriage. Ron Weasley, Ginny's brother, would also be important. He would be outraged and would probably be part of any plan to oppose the union.
Snape could also be a character in such a fanfic. His complex relationship with both Harry and Voldemort makes him an interesting addition. He might be in a difficult position, having to keep his cover as a double - agent while also trying to find a way to save Harry without revealing his true intentions to Voldemort.
Ron would also be important. His loyalty to Harry is unwavering. He might be the one who bravely volunteers to go on the most dangerous part of the rescue mission, perhaps facing off against some of Voldemort's most feared Death Eaters.