One plot twist could be that Harry's old enemies, like the remaining Death Eaters, find out about the marriage contract and try to use Gabrielle as a bait to get to Harry. This forces Harry to make difficult choices between protecting Gabrielle and fighting against his long - time foes. Meanwhile, Gabrielle might not be as helpless as everyone thought and could end up helping Harry in unexpected ways.
A possible plot twist could be that the marriage contract was actually forged by a dark wizard to gain control over Harry. So, when they start to discover this, they have to fight against the dark wizard's plans while also dealing with their growing feelings for each other.
Perhaps Gabrielle discovers that she has some hidden magical powers that are related to the marriage contract. This could change the dynamic between her and Harry completely. Maybe these powers are the key to saving the wizarding world from a new danger, but using them comes with a great cost. This would add a whole new layer of complexity to their relationship and the story.