What would be the plot if Harry and Ginny join Voldemort in fanfiction?

Yes, it is possible. By creating a complex backstory such as a powerful curse that makes them act against their nature, or a situation where they believe Voldemort is the lesser of two evils. For example, if the wizarding world has become corrupt in a different way than Voldemort's regime.

Harry could be used as a symbol of power. Voldemort might make Harry a sort of figurehead to show that even the 'Chosen One' has joined him, demoralizing the resistance. Hermione, on the other hand, could be forced to use her knowledge for Voldemort's dark magic research, but secretly she could be sabotaging it.

One possible plot could be that they are forced to join Voldemort under some sort of dark magic compulsion. Maybe Voldemort has a new powerful spell that he uses on them to make them his servants. Then they have to struggle with their own moral compasses while doing his bidding, and secretly try to find a way to break free and defeat him from the inside.

Some might write it to explore a different side of the characters. It's a way to show how Harry and Sirius could be pushed to their limits and what they would do in extreme situations.

One possible plotline could be that Harry is tricked by Voldemort. Voldemort might use some powerful dark magic or false promises to make Harry believe that joining him is the right thing to do. For example, Voldemort could claim to have a way to end all the bloodshed in the wizarding world if Harry helps him.

Yes, there are some. You can try searching on fanfiction websites like FanFiction.net or Archive of Our Own. Some stories might explore an alternate universe where Harry joins Voldemort for various reasons, such as being manipulated or having a different view of the wizarding world's power structure.

At first, Harry would be conflicted. He has always been against Voldemort, but now being forced to be with him would make him question his own beliefs. His character could become more cynical as he is exposed to Voldemort's dark world.

In a 'Harry and Sirius join Voldemort' fanfiction, it might start with Voldemort making a very tempting offer. For Harry, it could be something related to saving his friends or family. Sirius might be lured by the idea of protecting Harry in a different way. However, once they join, they would face a moral dilemma. They would have to pretend to be on Voldemort's side while still holding onto their own values. And there would be a lot of internal conflict as they see the evil deeds happening around them.

A plot could involve Ginny being the key to defeating Voldemort once and for all in a way that no one expected. Since she has a strong connection to Harry, and Harry was once a Horcrux of Voldemort, perhaps her magic has a unique ability to target and destroy the remaining essence of Voldemort. This could lead to a thrilling adventure where Harry and Ginny have to work together in a very different way than before to finally rid the wizarding world of Voldemort's threat.

Well, in this sort of fanfiction, Ginny's motive could be a complex web of emotions. She might be under the influence of a dark spell. Or perhaps she has been misinformed about Harry's role and believes that by giving him to Voldemort, she's actually saving the wizarding world. It could also be a case of extreme jealousy, if the writer wants to explore a really twisted version of her character.