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.
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 in a number of ways. For one, Voldemort might use Harry as a symbol to further his control over the wizarding world, claiming that even the 'Chosen One' has seen the error of his ways and joined him. Fred and George could be used to spread fear and chaos with their magical skills, but they might also try to use their position to gather intelligence for the resistance. Another aspect could be the reaction of the other characters. Hermione and Ron would be devastated and likely try to find a way to rescue their friends. The Order of the Phoenix would have to reevaluate their strategies completely.
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.
In such a fanfiction, Harry would grow in his understanding of the dark side of magic. Being captured by Voldemort means being exposed to the darkest and most malevolent forms of magic. This would make him more cautious in his own use of magic, but also more knowledgeable about how to counter dark magic. His character would also become more patient. He can't rush into things as he is in a very dangerous situation. He has to bide his time, wait for the right moment to escape or fight back. This new - found patience would be a valuable asset in his future battles against Voldemort. Moreover, he would develop a stronger sense of self - identity. He would be constantly reminded of who he is and what he stands for in the face of Voldemort's attempts to break him.
In the fanfiction, the howler could be from an unexpected source. Maybe it's from a long - forgotten enemy who has finally found a way to get to Voldemort. The howler's message could be a warning about a new threat that is coming his way, something that could even be more powerful than him. So Voldemort has to decide whether to ignore it or take it seriously.
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.
In a 'Harry Potter in a Custody Battle' fanfiction, it might be that Harry's muggle relatives and the wizarding community are in a custody battle. The Dursleys could claim they've been taking care of Harry all these years and have the right to keep him. But the wizarding world, like Hogwarts professors, might argue that Harry belongs with his own kind where he can learn magic and be safe from the threats only the wizarding world understands. There could be courtrooms filled with magic spells and arguments about what's truly best for Harry.
His character would likely be very different. He might be more cynical. Growing up with Voldemort, he would be exposed to a lot of dark and cruel things. He may start to question the concepts of good and evil as they are traditionally defined in the wizarding world.
In this fanfiction, the relationship between Harry and Voldemort would be full of contradictions. Voldemort adopts Harry for his own purposes, but Harry has his own will. In the beginning, Harry may be forced to follow Voldemort's orders out of fear. As time passes, he might start to question Voldemort's actions and ideology. Voldemort, seeing Harry's growing independence, could become more controlling. This could lead to a cycle of rebellion and suppression in their relationship. Harry may also find some common ground with Voldemort in their shared loneliness or desire for power, which would add more layers to their complex relationship.
It might start with Voldemort's initial curiosity. He realizes Harry is more than just a threat. So he begins to observe Harry closely. As time goes by, Voldemort may start to feel a strange sense of connection, like he's the only one who truly understands Harry's power.