In an interview with EW, Frozen producer Peter Del Vecho, revealed that in the original script of Frozen...Anna & Elsa weren't sisters! Elsa was supposed to be the film's main villain!
“We started out with an evil female villain and an innocent female heroine and the ending involved a big epic battle with snow monsters that Elsa had created as her army,” Peter explained.
The earlier version of the film would have started out with a prophecy that said “a ruler with a frozen heart will bring destruction to the kingdom of Arendelle.” Assuming that ruler was the main villain Elsa, but in the final scene Elsa was going to release a snow monster and Prince Hans triggers a massive avalanche to stop it. Yet that avalanche was going to destroy Anna and Arendelle. So after a heartfelt speech by Anna...Elsa uses her powers to stop the avalanche saving everybody. The twist being the prophecy was about Hans not Elsa!
Very interesting stuff...