The Frozen phenomenon knows no bounds, but this could be the Disney hit's most interesting crossover so far. In this week's season finale of Once Upon a Time, Captain Hook and Emma's time travelling brings back more than one surprise. In the final moments of the episode, we see a figure form from blue liquid and then turn into a beautiful blonde, complete with sparkly blue gown and braid: Elsa! The show's Facebook page also posted this little teaser image. We'll have to wait to find out what it all means, but it won't stop us from speculating wildly. How big will her role be in season four? Is she here to cause trouble? Does she just want to build a snowman? And most importantly, who will play her?