There could be a dark spirit that has been awakened by the little brother's emerging powers. This spirit is angry at being disturbed and decides to target the little brother. It causes chaos in Arendelle, blaming it all on the little brother. As the story progresses, the little brother has to find a way to either defeat or pacify this dark spirit to save his home and family.
Well, one possible plot could be that the little brother has some unique magical abilities that are different from Elsa's ice powers. Maybe he can communicate with animals in the enchanted forest and has to use this power to save the kingdom from a new threat.
It could be a jealous noble from the kingdom who wants to use the baby's power for his own gain. He might try to kidnap the baby and force it to use its powers for evil purposes.
Kristoff might also be a popular character in some fanfictions. He could be the one who helps Elsa and Anna on their adventures, providing his knowledge of the mountains and his loyalty. His relationship with Anna can also add an interesting dynamic to the story.
He could go on ice - climbing adventures with Elsa. Since Tarzan is so good at climbing trees in the jungle, he might adapt well to climbing ice cliffs in Arendelle. And with Anna, he could explore the forests around Arendelle, sharing his knowledge of the wild.
In some of these fanfictions, Sven could be a significant character. He might help Kristoff in his tasks related to Anna's pregnancy, like carrying supplies. Another character could be the trolls. They are known for their wisdom in the Frozen universe. So they could offer advice to Elsa and Anna about their pregnancies, such as how to deal with any magical complications or how to raise their children to be good and strong. And of course, the people of Arendelle are also important. Their reactions and support play a role in the story.
Well, in fanfiction, anything can be possible. Maybe it could be due to some magical cross - over event. Perhaps a powerful magic combined the different worlds of Tarzan, Elsa, and Anna, and as a result, Tarzan became their brother in this fictional scenario.
Kristoff is often involved. He would be there to support Anna, and might be a bit nervous about becoming a father. Olaf could also be a popular character in such fanfictions. He'd be his usual cheery self, trying to make the pregnant Anna and Elsa happy with his funny remarks.
One interesting plot could be Elsa and Anna going on a magical adventure outside of Arendelle. They might discover a hidden land filled with unique creatures and have to use their sisterly bond and powers to overcome challenges there.
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.
It could be Pitch Black. He might see the union of Jack and Elsa as a threat to his plans of spreading darkness. Their combined powers of ice and winter magic could be a powerful force for good, which he wants to prevent. So, he tries to create misunderstandings between them and stir up trouble in their relationship.