I've heard so many really amazing ones, but I'll only say my favorite:
Maybe my favorite movie theory, but applies to Broadway because the show is on Broadway now. In Frozen, that Hans may not actually be evil, and that he was cursed by the trolls. At the beginning of the show, he seems very genuine, and he could just be acting to be with Anna, but he seems to actually like her. Later in the movie/show in Fixer-Upper, there are lines of the song that seem very suspicious. The trolls are trying to set Anna up with Kristoff, but learn she's engaged to Hans, and they say things like: "[Her engagement]'s a minor thing", "Her quote engagement is a flex arrangement", and "get the fiance out of the way and the whole thing will be fixed." Don't these sound kind of fishy? Then when she goes back is when Hans really shows that he's evil, which is kind of unexpected. We know the trolls can alter memories as well.
IDK I just love this theory.