Elsa is too predictable, especially since she was almost evil in the original ending of the movie.

Hans is a dead no, as he was already a villain in the first movie. Unless he has some major plan, he wouldn't dare face Anna or Elsa again.

Kristoff would be a shocker if he turns bad, but I feeling that's not going to happen.

So, the real answer is obviously Anna, as no one would expect her to turn evil, especially since she saved Elsa at the end of the first movie. My guess would be, that she would still be herself in the first half of Frozen 2, but slowly discovers her own powers, which will be fire. Maybe just like with Hans showing his true colors, Elsa will fooled by Anna and eventually betrayed by her.

Anna would be a weird choice for the villain.