EDIT: All non-English sites have now been updated to correctly display the Year of the Dragon icon.
Thanks to an update on the official Hearthstone site that has been taken down, we've seen an icon that may be representative of the next year!
In a post on reddit, use25 discovered that the official site had a new icon in place of the Year of the Dragon icon on the "Ways to Play" page. As of posting, although it was removed on the English site, it is still available on the Russian site and other locales.
Us, and many others, believe this represents a Phoenix. You can make out the head and body in the middle of the icon with it surrounded by flames.
Year of the Phoenix Speculation
Speculating from a lore standpoint about possible themes for the year, it’s reasonable to say that at least one of the card sets will involve something related to Warcraft phoenix imagery, if this leak is in fact genuine—which I will personally take with a grain of salt until the official announcement confirms or denies it. All four previous Hearthstone years have had one expansion at least loosely connected to their animal. Year of the Kraken had the tentaclely Old Gods, Year of the Mammoth started with the prehistoric Un’goro Crater, Year of the Raven started with the spooky Witchwood, and Year of the Dragon needs no explanation.
Going off this assumption, the most likely theme related to a phoenix is the blood elves. Ruling in the north of the Eastern Kingdoms from their capital city of Quel'Thalas, the story of the blood elves is one of symbolic rebirth after they were exiled from the night elves centuries ago for refusing to stop practicing arcane magic. Known then as high elves, they took the name of blood elves after their city was destroyed by the undead Scourge under the command of The Lich King. With their city in ruins and most of their people dead, the survivors rallied together and took on the name of “blood elves” to honor their fallen kin. The phoenix is their primary symbol—seriously, it’s everywhere—and a set about blood elves could very well be possible after the sets we’ve got focusing on worgen and troll cultures.
Even their magic staffs are designed in a phoenix shape: the open mouth in the center of outstretched wings.
Another less likely possibility is an elemental-focus set, as phoenixes are fire elementals. However, seeing as fire elementals have already had a center stage position in Blackrock Mountain, and elementals themselves were already a big part of Un’goro, it’s unlikely they will be the focus again so soon where there are many other topics to cover and places from Warcraft lore to explore.