I don't like spreading the rumor mill but chances are you all will here about this eventually. This is a quote from Dulfy, "Guild Wars 2 Heart of Thorns has been trademarked by Ncsoft and will likely be the name of the first ever expansion for GW2." Keep in mind this is merely an opinion and does not mean this name will lead to an actual expansion.
More information can be found here (image is of Colin Johanson, Game Director for GW2):
http://dulfy.net/2015/01/13/gw2-ncsoft-trademarks-gw2-heart-of-thorns/