10 Movie Fan Theories EVERYONE Believed
6. Anna Has Fire Powers - Frozen II

The Theory
The sequel will reveal that Elsa's (Idina Menzel) young sister Anna (Kristen Bell) has magic powers of her own, but running counter to Elsa's ice powers, they're fire.
Why Everyone Believed It
Thematically, it makes total sense to oppose Elsa and Anna's abilities in yin-and-yang fashion like this, while the film's trailers made numerous memorable mentions to fire (such as "You don't want me following you into fire? Then don't run into fire!").
Given that we knew the film would center heavily around the elemental spirits, it'd be a neat way to move Anna forward as a character and fundamentally change her relationship with Elsa.
What Actually Happened
Anna exhibits no supernatural abilities whatsoever in Frozen II and becomes the new Queen of Arendelle at the end of the movie. As for the fire spirit? It actually turns out to be an adorable little salamander named Bruni.