In many ways Frozen represents the new era of Disney, but the modern megahit has actually been in the works since before you were born. Just before the release of Snow White and the Seven Dwarfs in 1937, Walt himself was looking into an adaptation of Hans Christian Anderson's The Snow Queen. Over the course of nearly 70 years, this movie became Frozen -- but the original Danish fairy tale is almost unrecognizable compared to the adventures of Anna and Elsa.
Part of the problem Disney ran into initially was that The Snow Queen is a pretty dark story. It begins with an evil mirror shattering, its shards finding its way into a young boy named Kay, who is corrupted by its black magic. Kay is then kidnapped by the Snow Queen and kissed two times -- the first time to help him withstand the cold, the second time to brainwash the boy into forgetting his family. It is said that a third kiss would kill Kay outright, so basically the Snow Queen has a gun to her captive's head for the entire story. Only through the brave actions of Kay's young neighbor Gerda is the Snow Queen thwarted by, what else, the power of love.
Of course, Disney fans know that tons of the original fairy tales were even more disturbing in their own right -- what with the original Ariel dying of a broken heart and dissolving into seafoam. The characters themselves were also an issue, specifically the Snow Queen, who proved to be tough to portray as a likable (or at least watchable) personality. For a while there, the closest Disney came to a real adaptation was in the late 90s. This was back when the movie was still going to be 2D, hand-drawn animation.
This version was at one point known as "Anna and the Snow Queen" (which oddly enough, is what the Japanese title for Frozen translates to). The little we do know about the story at this stage comes from concept artists like Mike Gabriel. In AatSQ, Anna finds a buried ice palace in the wilderness, a place thought only to be a legend. The thrust of the movie would begin when Anna accidentally ressurects the Snow Queen, who had been frozen in a block of ice. The two weren't sisters, and presumably didn't sing adorable but depressing songs about building snowmen.
Un-Elsa herself was also much different; the concept artist described her as a "Vegas showgirl type," which you can see above. Though Josh Gad was still in the picture as an early form of Olaf, Megan Mullally was set to play the Snow Queen instead of Idina Menzel; avid TV-watchers might know Mullally as Karen on Will & Grace or Tammy 2 on Parks and Recreation. Though Sexpot Elsa obviously didn't make it to the final product, if you want to see more of that, well, you're already on the internet.
As we see from Claire Keane's concept art, the Snow Queen maintained a kind of rockstar/showwoman design for some time. On the left you can see Bette Midler's influence on Keane's art, and the image on the right is inspired by Amy Winehouse.