It really shouldn't come as any surprise that a Frozen sequel is possibly underway, as the hit animated film has made over one and a quarter billion worldwide and is the highest grossing film of all time. According to the Telegraph, Idina Menzel, who played Elsa, admitted that Disney is "already in the works" on a sequel.
There is already report of a short film to be released next year, according to Enstarz. The short film could be out as early as Spring 2015, with a title of "Frozen Fever". Imdb's page about the short film reports that the plot of this sequel will be Anna's birthday party, and the fear that Elsa's power could spoil the party.
If you are looking for an actual Frozen 2 full-length movie, you might have to wait years. Disney's animated films, even with their advanced computer-generated technology, take years to make. This means that projecting a release date has to be done years in advance. Right now, Disney already has movies in the pipeline with Moana and Zootopia, films that won't show up until 2016, not to mention Inside Out for 2015, done with cooperation with PIXAR.
Unless Disney was certain on Frozen being the huge hit that it was, they would not have ordered up sequels without testing the original. This means that unless Disney drops all of its current animated projects, the earliest one can expect a Frozen 2 would be in 2017 or 2018. So far, the rumored plotline for Frozen 2, according to KDrama News is that Elsa will be getting a love interest, since Anna has Kristoff. Another rumor is that Chris Buck and Jennifer Lee, co-directors and writers of the original Frozen, have tossed names around of actors for the sequel which included Jamie Foxx, Melissa McCarthy, Benedict Cumberbatch, Matthew McConaughey, and Judi Dench.
The original Frozen film was left open for a sequel as two of its villainous characters Hans and the Duke were still alive and being taken away from Arendelle. Considering they had left a place with a queen who could freeze an island at will, these characters could easily end up working together to try and usurp her. After all, Elsa has the power to make living snowmen, which means that she could make an army and attack any country. You can argue that Hans and the Duke could convince many other countries in the Frozen universe to try and take Elsa out. This type of plotline would definitely lend the opportunity for guest stars.
Until "Frozen Fever" and Frozen 2 comes out, you can still see both Anna and Elsa on Once Upon A Time, as they are recurring characters, Sundays on ABC.