Perhaps the biggest plot twist would be Harry's view of the Death Eaters. Growing up with the knowledge that his mother was Bellatrix, he might have a more sympathetic view of some of them at first. He could believe that they were misled rather than inherently evil. This would lead him on a journey to try and reform some of the Death Eaters, which would put him at odds with both the Order and the remaining Death Eaters who don't want to change. It would also change the way we see the divide between good and evil in the wizarding world.
The plot could involve the son trying to redeem his mother's name. He might go on a journey to find out more about her past, hoping to understand why she became so evil. Along the way, he meets various characters who knew his mother and has to deal with their mixed reactions towards him.
A major plot twist could be that the father of the child is a secret enemy spy. This would add a whole new layer of drama as Bellatrix's daughter has to deal with the betrayal and also protect her child from both sides. Another could be that the pregnancy is part of a magical prophecy that she was unaware of, and it has far - reaching consequences for the wizarding world.
Another plot twist could be that Bellatrix realizes she has created a powerful adversary in Harry. As she raises him to be Lord Black, she instills in him a sense of power and ambition. But Harry uses that power not for the dark purposes she intended. Instead, he decides to reform the Black family traditions and bring a new era of inclusiveness to the wizarding world.
A plot might revolve around the wizarding community's reaction to Harry's new identity. Some might fear that he has the same dark tendencies as the Lestrange family. This could lead to Harry being ostracized at Hogwarts. Meanwhile, he might start to notice some strange powers or abilities within himself that he can't explain. He then tries to find out if these are related to his Lestrange heritage. Along the way, he forms new alliances with some unlikely characters who are also interested in the secrets of his family.
One possible plot twist could be that Harry struggles with his new identity. He might initially reject the idea but then gradually discover certain dark powers within him that he has to learn to control. This could lead to a conflict with his friends who are unsure whether to trust him.
One possible plot twist could be that Harry's connection to Narcissa changes his relationship with Draco. Instead of being rivals, they could be reluctant allies, with Draco feeling a sense of family loyalty towards Harry. Another twist might be in how the wizarding world reacts. Some might suspect that Harry is now a part of some dark plot due to his Black family connection. And the magical abilities he inherits from the Black family could be different from what he has as a Potter, leading to new challenges and adventures as he tries to master them.
She might be motivated by a long - lost connection. For instance, if there was a family tie between Harry and her that she was previously unaware of. Or perhaps she has a change of heart after seeing the true nature of Voldemort's actions and decides that Harry is the only hope to stop the chaos and destruction Voldemort is causing. It could also be that she is magically bound to save Harry due to some ancient spell or prophecy that she discovers.
Perhaps a plotline where Harry discovers the truth about his parentage later in life. He has been raised believing Bellatrix is his mother, but then uncovers evidence that it's all a complex web of dark magic and deception. This could lead to a journey of self - discovery as he tries to figure out who he really is and where he belongs in the wizarding world, all while dealing with the stigma of being associated with Bellatrix Lestrange.
The plot could take a turn where Pan Son is initially thought to be a normal Muggle, but as he grows up with Harry, it's revealed that he has a dormant wizarding bloodline. This discovery could come at a crucial time, like when Harry is facing a great threat. Pan Son's newfound magic could be the key to turning the tide. They would then have to train Pan Son quickly and figure out how to integrate his magic into their fight against the dark side.
One possible plot could be Harry growing up believing he is on the right side by following Bellatrix and the Death Eaters. But then he discovers some secrets about his real parents and starts to question everything he's been taught. This leads to an internal struggle as he tries to decide whether to stay with Bellatrix or break away and find his true identity.