Another possibility is that it could be a character from another Disney universe. For example, a prince from a neighboring magical kingdom. This would add an interesting crossover element to the fanfiction and create a whole new dynamic for Elsa's pregnancy story.
Perhaps it could be Kristoff. Although he was more interested in Anna in the original story, in fanfiction, the relationships can be reimagined. They could have developed a closer bond over time and he could be the father.
Maybe it was someone from Elsa's past that we don't know much about yet. For example, a childhood friend who she had lost touch with but who returned to her life. They rekindled their friendship which then turned into a deeper relationship, resulting in her pregnancy.
Well, in fanfiction, the rules can be bent. For Elsa to be pregnant, it could be part of a 'what if' scenario. What if Elsa had a different upbringing and was more open to relationships earlier? This could lead to a situation where she becomes pregnant. It might also involve some sort of prophecy or magical destiny that was previously unknown in the Frozen story, which now unfolds in the fanfiction.
A common theme is also about acceptance. The people of Arendelle need to accept the fact that their queen is going to be a mother. And Elsa herself has to accept the changes in her body and life. In addition, the theme of love is often present. Whether it's the love for her unborn child or the love from her friends and family during this time.
Another option could be a character from a different fictional universe in crossover fanfictions. For example, a character like Jack Frost from Rise of the Guardians. Writers might bring these two together because they both have powers related to ice and snow, and it could make for an interesting dynamic as they explore their powers together and develop feelings for each other.
It could be a power - hungry villain. Someone who sees Elsa's powers as a means to gain more control. Maybe a dark wizard or a warlord.
It's possible that there are some out there. But keep in mind that a lot of fan - created content in the 'Frozen' fandom is centered around Elsa's growth as a character, her journey to accept herself, and her role in Arendelle. While pregnancy is a normal part of life, it might not be the first thing that comes to mind when thinking about Elsa in fanfic writing. Also, Disney has its own image of Elsa that some fanfics may or may not adhere to.
Another theme is the relationship between Elsa and the other characters in Frozen. For example, how Anna supports her sister during the pregnancy. It can also focus on Elsa's relationship with Kristoff and Sven, and how they adapt to the idea of a pregnant Elsa.
One common theme is the exploration of Elsa's new role as a mother - to - be. It often involves how she deals with the changes in her life and her powers in relation to her pregnancy.
It could be a vampire lord who has a vendetta against Elsa's family. He believes that they have something that belongs to him, and when he discovers Elsa, he decides to use her as a pawn in his scheme. This vampire lord is cruel and calculating, and he has an army of minion vampires at his disposal. He uses his charm and power to manipulate those around Elsa and tries to isolate her so that he can achieve his goals. He is also very strong and has abilities that make him a formidable foe for Elsa.
One possible antagonist could be a jealous sorcerer. He might be envious of Hiccup and Elsa's relationship and the power that their unborn child is said to possess. So he tries to harm Elsa during her pregnancy to stop the child from being born. Another could be some of the traditionalists from their respective kingdoms. They don't approve of their relationship and see the pregnancy as a threat to their old ways. They might try to separate Hiccup and Elsa, which would put Elsa's pregnancy at risk.