One possible plot twist could be that the Death Eater she marries is secretly a double agent. He might be working against Voldemort from within the Death Eater ranks and Hermione discovers this. Their relationship could start as a way to get closer to the Death Eater secrets but then turn into real love.
Another plot twist could be that it's all a part of a time - travel experiment gone wrong. Hermione gets sent back in time and ends up marrying a Death Eater in that era. But as she tries to find a way back to her own time, she starts to have feelings for him and has to decide whether to stay or go.
Another aspect could be an unexpected bond forming between Hermione and the Death Eater. As they are forced to spend time together, they might start to understand each other on a deeper level. This could lead to a moral dilemma for the Death Eater, especially if he starts to develop feelings for Hermione. He would then have to choose between his loyalty to the Death Eaters and his new - found connection with her.
Perhaps Hermione's transformation into a Death Eater is a result of a time - travel accident. She goes back in time, something goes wrong, and she ends up being influenced by the Death Eaters of that era. This would create a complex web of cause and effect in the fanfiction.
Well, it could be that Hermione is under a spell when she marries Rowle. The fanfiction could then revolve around how she breaks the spell and what she does after realizing she's been forced into this marriage. She might find that Rowle has some redeeming qualities despite the situation, and they could either part ways amicably or decide to work on a real relationship.
One potential plot twist could be that Hermione's father was actually a double agent. So, while everyone thinks she's the daughter of a Death Eater, in reality, he was working for the good side all along. Another twist might be that Hermione discovers she has some special powers because of her heritage that can be used to defeat Voldemort in a unique way. For example, she could have an innate understanding of dark magic that helps her counter the Death Eaters' spells.
A great plot twist could be that the Death Eater she's betrothed to is actually a double agent himself. All along, Hermione thought she was the one with the upper hand in the undercover operation, but it turns out he's been playing a long - game too. This would completely change the power dynamic between them and add a whole new layer of complexity to their relationship.
A possible plot twist could be that Hermione gets turned into a Death Eater against her will, and Draco has to help her break free from the dark magic controlling her. This would completely reverse their typical roles.
It could be about Hermione facing challenges from a Death Eater while dealing with the slur of being called 'mudblood'. Maybe the Death Eater tries to capture her for some dark purpose, and Hermione has to use her intelligence and magic skills to outwit the enemy.
It could be about Hermione somehow influencing a Death Eater to change their ways. Maybe she uses her intelligence and magical skills to show the Death Eater the error of their beliefs. For example, she might expose the true nature of Voldemort's regime which is based on fear and oppression, making the Death Eater question their loyalty.
Bellatrix Lestrange is also a likely candidate. Her encounter with Hermione would be extremely intense. Given her loyalty to Voldemort and her rather unhinged nature, she would likely pose a serious threat to Hermione, but Hermione might also find a way to use Bellatrix's madness against her.
One possible plot could be that Hermione discovers a Death Eater who has doubts about Voldemort's cause. Maybe the Death Eater has a family member who was unjustly punished by Voldemort. Hermione, being the intelligent and empathetic witch she is, offers the Death Eater a way out. She could provide a safe haven or help them fake their death and start a new life away from the Dark Lord's influence.