Power struggles can be a theme as well. If the OC has some sort of power, like being a powerful vampire or a witch with unique abilities, there could be power struggles between them and Elena Gilbert. Or they could team up to face power - hungry villains in the show's universe. This adds an interesting dynamic to their relationship and the overall story.
One common theme is friendship. The OC might start as a friend to Elena Gilbert and then their relationship could develop into something more. Another theme is adventure. They could go on adventures together in the supernatural world of the Vampire Diaries.
One great place to start is Archive of Our Own (AO3). There are numerous Elena Gilbert x OC fanfictions there. You can search for ones with high ratings and a lot of kudos. Some popular tags to look for might be 'alternate universe' or 'romance'.
One common theme is the struggle between their human and supernatural sides. Both characters have to deal with being part of the supernatural world in their original shows, and this often comes up in fanfiction.
Love and friendship are common themes. Their relationship starts as a friendship with a bit of rivalry and then blossoms into love, which is often explored in fanfictions.
In violet x gilbert fanfiction, the theme of acceptance is quite prominent. Gilbert accepts Violet for who she is, with all her quirks and past traumas. And vice versa. Another theme is the power of connection. Their relationship is depicted as something that gives them strength. And the theme of hope also runs through many of these fanfictions, as their relationship is often a source of hope for both of them in a rather difficult world.
Friendship is a very common theme. It often shows how their bond grows and endures different trials.
It could lead to a completely different plotline. Maybe she'd have wild mood swings and unpredictable actions.
One common theme is the struggle of Jeremy dealing with the new responsibility. He might be worried about protecting the reader and the unborn child from the dangers in their world, especially if it's in the 'The Vampire Diaries' universe with all its supernatural threats.
A common theme is the redemption of their past. Both Gilbert and Violet have had difficult pasts, with Violet being used as a weapon and Gilbert having to endure the hardships of war. Fanfictions often explore how they heal each other. Another theme is the discovery of love. In the anime, their love was a bit ambiguous at times, and fanfictions clarify and intensify it.
Overcoming obstacles is also frequently seen. Whether it's social differences, misunderstandings, or external problems, they have to work through these issues to be together. This adds drama and depth to the fanfics, making their relationship more interesting to read about.
One common theme is power dynamics. Since Rumpelstiltskin is a powerful character in the fairy - tale, when paired with an OC, the story often explores how their different levels of power interact. For example, if the OC is a meek character at first, they might grow in power or learn to navigate around Rumpelstiltskin's power.