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.
Yes, there might be some. However, it's a rather dark and unorthodox take on the Harry Potter universe. Some fanfictions like this explore extreme 'what - if' scenarios. But they often deviate greatly from the original spirit of the series.
One common theme is power struggle. Since Harry has his own power and Voldemort is power - hungry, there will be a constant fight for control over Harry's abilities. Another theme is the loss of innocence. Harry, being in such a dark situation, would surely lose his naivete quickly.
I'm not sure of a specific title off - hand, but in such fanfictions, usually, the story would show how Harry's life changes drastically. Maybe he has to endure a lot of hardships under Voldemort's control, or perhaps he tries to find a way to escape and turn the tables on Voldemort. It's a very dark and interesting concept that can be explored in many ways.
Another plotline might be their family life. They could be shown starting a family early, and dealing with the joys and challenges of raising children in the wizarding world. For example, teaching their kids magic at home, or dealing with the chaos of a family full of young wizards.
There's a fanfic called 'Ginny and Harry: A Love Unhindered'. In this story, they start dating right after the defeat of Voldemort's minions in their final year at Hogwarts. Without the threat of Voldemort, they are able to fully explore their feelings for each other. They go on adventures across the wizarding world, from the beaches of Cornwall in the wizarding UK to the magical markets in Paris, just the two of them.
This is a very strange and dark concept for fanfiction. In a possible plot, perhaps they are under some powerful mind - controlling spell. Maybe Voldemort has found a way to use their skills and knowledge for his own evil ends. For example, Harry's connection to the Deathly Hallows could be exploited, and Ginny's powerful magic, which was shown in the books, could be used to carry out Voldemort's orders against their will.
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.
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.
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.