Now that we have confirmation that the upcoming fall set is indeed the war-themed, Mongolian-esque Khans of Tarkir, all eyes are on the one confirmed planeswalker so far: Sarkhan. As Sarkhan was speculated to be in this set ever since the first trademark (Warlords of Khanar) for this set was announced based simply on the name similarity, his presence is hardly news.
What is interesting, however, is his physical appearance, and the current path of his story arc. Last we heard from Sarkhan, he was on Zendikar, helping to instigate the release of the Eldrazi from their hedron prisons. All of this he was doing under the influence of Nicol Bolas, and also not in his right mind. Sarkhan, this time around, looks much more collected and purposeful in his actions. He's ditched some of the darker accoutrements of his previous costume, switched his nasty-looking polearm for one that's more reasonable and friendly, and his eyes now glow a fiery, composed orange instead of a sickly red. Bolas was able to manipulate Sarkhan because Sarkhan was lost, but now that he's home the Dragonmaster has a greater aim than serving the elder planeswalker.
This means he's probably the main protagonist of Tarkir block, and also that he's undergone a color change. Whether he's back to being only red and green or is now black, red, and green is hard to say (though that'd make this the third block in a row with a red-green 'walker and make Sarkhan the fourth planeswalker in that color combination printed, which is rather disproportionate). However, Sarkhan is on Tarkir for a reason - he is, no matter how you slice it, integral to the plot. And the fact that Sarkhan is a Dragon-worshiper and the Dragons of Tarkir are all dead, coupled with the undead one seen in the teaser artwork, makes it seem as though that particular type of creature is the focus. Either Sarkhan seeks to revive the long-gone Dragons and restore them and the plane to their former glory, or he's trying to stop our villain - presumably a necromancer - from desecrating his idols by forcing their dead bodies into servitude.