Anna would be a likely candidate to rescue Elsa. She loves her sister deeply and has shown great courage in the past. She would stop at nothing to find and free Elsa from Hans.
The villain could be a jealous ice - mage who wants Elsa's powers all to himself. He sees Elsa as a threat to his own dominance in the world of ice magic.
Well, one possible plot could be that Hans has some hidden motives for kidnapping Elsa. Maybe he wants to use her powers for his own gain, like ruling Arendelle through her magic. He could hold her in a secret location and try to force her to cooperate.
One plot idea could be that Elsa is kidnapped by an evil sorcerer who wants to use her ice powers for his own dark deeds. The heroes from her world then have to embark on a journey to rescue her. Another could be that Elsa is kidnapped by a group who think her powers are a threat and they plan to keep her locked away forever. However, her sister Anna discovers her whereabouts and tries to free her.
One possible plot could be that Jack Frost, perhaps out of loneliness or a misunderstanding, kidnaps Elsa. He takes her to his icy realm. Elsa is initially scared but then realizes Jack has his own insecurities. They start to talk, and Elsa, with her power over ice, shows Jack new ways to use his powers. Eventually, they become friends and Jack realizes his mistake and returns Elsa to Arendelle.
Kristoff might also save Elsa. He is used to the cold and has his reindeer Sven to help him. He would follow any leads to find Elsa. And with his knowledge of the icy terrains, he could track down Jack Frost's hideout and rescue Elsa.
One idea could be that a powerful dark sorcerer kidnaps Elsa because he wants to use her ice powers for his own evil deeds. He locks her in a magically sealed dungeon deep in an ancient castle. Another plot could involve a group of thieves who mistake Elsa for a valuable princess they can ransom. They take her to a hidden lair in the mountains. A third option is that a jealous rival from another kingdom, who envies Elsa's powers and her popularity, arranges the kidnapping to discredit her.
She could interact with Daenerys Targaryen. They are both strong female characters with special powers. They might initially be rivals, both vying for power and influence in Westeros, but then realize they can achieve more by working together. For instance, Elsa could use her ice powers to cool down the hot deserts that Daenerys has to cross with her armies.