If Voldemort gets custody of Harry in fanfiction, it would be a huge deviation from the original story. Harry would probably be exposed to a lot of dark and cruel things. Voldemort could try to break Harry's spirit and make him abandon his values. There might be a struggle within Harry between the good he has been taught and the evil that Voldemort represents. And the wizarding world would be in even more chaos as Voldemort would have more control over the one person who was seen as the hope to defeat him.
The plot could develop with Voldemort trying to use Harry to open the Chamber of Secrets again, believing that there are secrets within that could enhance his power further. Harry, on the other hand, might find allies among the Death Eaters who are not fully loyal to Voldemort and use them to create a revolt from within. Also, Dumbledore and the Order of the Phoenix would surely be trying to rescue Harry, which could lead to many intense battles and rescue attempts.
In such a fanfiction, Harry might be used as a bait to lure his friends into a trap. Voldemort could try to extract information from Harry about the Order of the Phoenix or Dumbledore's plans. Maybe he would also attempt to turn Harry to the dark side by showing him the so - called 'power' of the Dark Arts.
In this kind of fanfiction, one possible scenario is that Harry's whole view of the wizarding world would be warped. Voldemort might try to mold Harry into his own image. He could teach Harry dark magic instead of the traditional magic taught at Hogwarts. Harry might grow up with a completely different set of values and powers, which would surely change the whole dynamic of the fight against good and evil in the wizarding world.
In some fanfictions, if Harry Potter gets broken by Voldemort, it could mean his spirit is crushed. Maybe he loses his will to fight and his friends have to find a way to bring him back. It might involve them using powerful magic or memories to reignite his courage.
Maybe Sirius gets help from some powerful wizards or witches who believe in his innocence. They might testify on his behalf in front of the wizarding authorities. Sirius also could show his strong emotional bond with Harry. He has always been there for Harry in spirit, and now he can be there physically to raise him, which convinces the authorities to give him custody.
The idea of Voldemort getting Harry pregnant goes against the original nature and themes of the Harry Potter series. J.K. Rowling's work is mainly about good vs. evil, friendship, and the fight against dark magic. So, while there might be some extremely non - canonical and out - there fanfiction on this very strange and inappropriate topic, it is not something that is in line with the spirit of the original story.
Well, if Harry is captured by Voldemort in fanfiction, it could lead to some really dark and intense scenarios. Voldemort might torture Harry to get information about the resistance. Harry, on the other hand, could be secretly planning an escape while pretending to be broken. There could also be a rescue mission by his friends, which would be full of danger and drama.
In such a fanfiction, Harry might become a very different person. He could be more in tune with the Dark Arts. Maybe he would have a skewed sense of morality, influenced by Voldemort's views. He might also be more powerful in the Dark Arts but struggle with his identity as he is constantly in conflict between what he's been taught by Voldemort and his own inner goodness.
In such a fanfiction, Harry might be raised with a very different set of values. Voldemort could try to mold Harry into his own image, perhaps teaching him dark magic. But Harry might also secretly hold on to his own sense of good. For example, he could use the knowledge Voldemort gives him to ultimately turn against him.
Perhaps James has more resources to take care of Harry. He could have a better living situation, more financial stability, or a stronger support system. In fanfiction, this can be a way to create a new dynamic for the characters, with James being the sole provider and protector for Harry.