A really interesting plot twist could be that Snape was under a spell all along that made him seem so cold and distant, and Hermione discovers it. This changes the whole dynamic between them. Another twist could be that Hermione is actually related to someone from Snape's past in a way that they never expected, which brings them closer.
One great plot twist is when it turns out that Snape has been protecting Hermione all along, but in a very secretive way. And when Hermione finds out, it completely alters her view of him. Also, there could be a plot twist where the reason Snape and Hermione are brought together has something to do with a time - traveling incident. They end up in a different era where they have to rely on each other and that leads to their relationship and marriage in the fanfiction.
Perhaps a plot twist where Snape is not fully human, like he has some kind of magical creature blood in him that only Hermione discovers. This revelation makes their relationship take a new turn. Or, there could be a plot twist where Hermione loses her memory and Snape is the only one who can help her regain it. During this process, they fall in love and get married in the fanfiction.
One popular plot twist could be that Snape is actually controlling Hermione to protect her from a greater danger that she's unaware of. Maybe there's a dark wizard targeting her and Snape's control is a way to keep her out of harm's way while he figures out how to defeat the threat.
Well, a really interesting plot twist could be Hermione's role in the Slytherin house. She could become a leader, using her intelligence to reform the house's more negative aspects. Or, she might be caught in the middle of a power struggle between different Slytherin factions. Also, her interactions with the Slytherin pure - blood families could be full of drama. Maybe she discovers a family secret that could change the whole wizarding world.
One cool plot twist is when Hermione discovers that Edward has a hidden power that is related to her magic. Maybe he can absorb and use some of her magic energy, which leads to a whole new set of challenges and adventures as they try to understand and control this new aspect of their connection.
In some fanfictions, the plot twist is that Hermione's magic has a different effect in Middle - earth than it does in the Harry Potter world. For instance, her spells might accidentally cause a rift between different races in Middle - earth, and she has to find a way to fix it while also dealing with the main quest of the dwarves to reclaim Erebor. It adds a whole new layer of complexity to the story.
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 plot twist could be that Snape is actually under a spell that makes him fall in love with Hermione. This could lead to a whole mystery about who cast the spell and why. Hermione would then have to figure out whether the love is real or not, while still dealing with the feelings she has developed for Snape. Another could be that they get married in secret, and when it's revealed, it causes an uproar in the wizarding community, with people taking sides either for or against their union.
A great plot twist could be that younger Snape actually has some knowledge of Hermione's time - traveling. Maybe he's been having strange dreams about the future and when he meets Hermione, he realizes she's part of it. This would completely change the power dynamic between them.
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.
One interesting plot twist could be that Snape, despite his initially cold exterior, uses his advanced potions knowledge to secretly develop a cure for Hermione's head trauma. This would add a layer of complexity to his character as it shows a more benevolent side.
One interesting plot twist could be that Severus Snape, as a 'king' in the fanfiction, discovers a long - lost magical art that gives him the power to change the past. This could lead to him preventing some of the tragedies that happened in the original Harry Potter story, like Lily's death. Another could be that he rules over a hidden magical realm where the rules of Hogwarts and the wizarding world as we know it don't apply, and he has to constantly defend this realm from dark forces that want to take over.