In some fanfictions, the soulmate marks might be used to show a hidden connection between them that goes beyond their enmity. It could be a way to explore the concept that they are two sides of the same coin. The marks could also be a source of power for both of them, either by enhancing their existing magic or unlocking new abilities that are only accessible when they are in close proximity to each other.
The soulmate marks can create a sense of confusion. Harry, being the hero, might be conflicted about having a connection to Voldemort. It could make him question his own identity and purpose. For Voldemort, it might be a source of irritation or an opportunity to manipulate Harry.
One interesting concept could be unique designs of the soulmate marks. For example, some fanfictions might have the marks be in the shape of a Hogwarts house crest. Another concept could be how the marks interact with magic. Maybe the marks glow when the soulmates are near each other and can even enhance their magical abilities when they touch. Also, the discovery of the soulmate through the mark can lead to all sorts of adventures and relationship developments in the wizarding world.
One idea could be that Harry, as Voldemort's soulmate, tries to use their connection to redeem Voldemort. He might show Voldemort memories of love and kindness in an attempt to change his dark ways.
Soulmate marks can speed up the process of relationship building. Since the characters know they are soulmates because of the marks, they might be more open and trusting towards each other from the start. It gives them a sense of security and destiny in their relationship.
One idea could be a fanfiction where Voldemort is redeemed. Maybe after a near - death experience, he has a change of heart and tries to make amends for all the evil he has done. He could start by secretly helping Harry Potter against some other dark forces that emerge in the wizarding world.
A very frequent theme is the exploration of destiny. The fanfictions often question if their being soulmates was fated all along. Also, the theme of internal conflict is common. Harry has to grapple with the fact that his soulmate is the most evil wizard, and Voldemort has to face his connection to the 'Boy Who Lived'. It makes for some really intense character development. For example, in some stories, Harry tries to find the good in Voldemort because of their bond, while Voldemort is forced to re - evaluate his hatred for Harry. This leads to a lot of interesting plot twists and turns.
Another plot could be that they are married in an alternate universe where their backstories are different. Maybe Voldemort was never as consumed by dark magic from the start. Harry and Voldemort could then discover that they have a deeper connection than they thought, and their marriage could be a journey of self - discovery for both of them, as they explore their relationship in a world that is both familiar and yet very different from the one they knew in the original story.
One idea could be that Harry starts having strange dreams where he sees through Voldemort's eyes. This could lead to him unknowingly helping Voldemort in some small ways at first, like revealing the location of some minor Order of the Phoenix safe houses. Another plot could involve the other characters slowly realizing something is wrong with Harry. For example, Hermione might notice that Harry is using some Dark Magic spells without hesitation, which is completely out of character for him.
In some fanfiction, there could be a new kind of pet that is created from the combined magic of Harry and Voldemort, perhaps accidentally. This pet has powers that are both light and dark. It could be a small creature like a mouse that has the ability to travel between the realms of the living and the dead. Harry and Voldemort then have to compete to gain the loyalty of this unique pet which holds great power.
A plot could involve a magical creature that bonds Harry and Voldemort in an unexpected way. For example, a rare faerie that makes them see each other's memories. Through this, Harry discovers some long - hidden secrets about Voldemort's past that could potentially change the course of their enmity. They then have to decide what to do with this new - found knowledge.