One common theme is friendship. Often, the OC becomes a friend to Elsa, helping her through her struggles with her powers and her role as a queen. For example, in many stories, the OC has a positive influence on Elsa's self - acceptance.
One great Elsa OC fanfiction is 'Frozen Hearts, New Beginnings'. It explores a new character in the Frozen world and their relationship with Elsa. The story has really interesting character development and beautiful descriptions of the Frozen landscapes.
Anna would surely be the first to help Elsa. Anna has always been there for her sister, and when Elsa gets sick, she would do everything she could, like bringing her warm soups and staying by her side to keep her company.
In this fanfiction, Marshmallow could be a significant character. He's a large and somewhat intimidating snow creature created by Elsa. But with the baby on the way, he could become a gentle protector. He could stand guard outside their home to keep any potential threats away. Also, the trolls from the Frozen universe could play a role. They are wise and could offer their knowledge on how to take care of a baby with magical powers. They might even perform a special ceremony for the unborn baby.
Their relationship would likely become more solid. Having a baby together means they have a new shared responsibility. They would have to rely on each other more, like when taking care of the baby at night or making important decisions about the baby's future.
One possible plot could be that Hermione, being a brilliant witch, discovers a magical portal that leads her to the world of Arendelle. There, she meets Elsa and is immediately intrigued by Elsa's ice powers. She decides to study them from a magical perspective. Along the way, they become friends and Hermione helps Elsa better understand and control her powers using her knowledge of magic and spells.
Their relationship might start with curiosity. Hiccup would be curious about Elsa's ice powers and Elsa about Hiccup's connection to dragons. As they interact more, they start to respect each other's abilities and personalities. This mutual respect turns into friendship, and then gradually into love. The news of the baby would strengthen their bond as they now have a shared future to look forward to. They would support each other more, with Hiccup protecting Elsa during her pregnancy and Elsa using her powers to help Hiccup in his adventures.
They can develop their relationship through their shared care for the baby. For example, when the baby is sick, Elsa uses her powers to keep it cool while Jack tries to cheer it up. This mutual effort in taking care of the baby can bring them closer.
It becomes deeper. They become more reliant on each other for emotional support. Since they are both going through the same experience of pregnancy, they can share their fears, joys, and concerns. For example, they might talk about how their bodies are changing and how they will raise their children.
Moana would probably be very curious about Elsa's ice powers. She might ask Elsa to show her how she creates ice sculptures. Elsa, on the other hand, would be interested in Moana's fearless attitude towards the ocean. They would start by sharing their different cultures. Moana would tell Elsa about the traditions of her island and Elsa would share about the life in Arendelle.