Following the trend that has been established, we should be expecting a White Planeswalker. However Sarkhan Vol is on Zendikar guarding the Eye of Ugin. I believe that he will become White since he is the only remaining Planeswalker, and the Eye of Ugin is powerful enough to do that. In one scene of the comic, Chandra is consumed by white fire but is still Red. Perhaps this is done by Sarkhan Vol? However, many other people believe that he will be Grixis colors since he has started losing his mind and is a slave of Nicol Bolas. I think that if he has gotten so bad from insanity, the Eye of Ugin would heal him so that he would be able to protect the Eye of Ugin better.
But would Wizards really break the color wheel and not make him White? And who would be left to fill the void of the White Planeswalker slot? A friend of mine speculated that Elspeth would come to Zendikar, but why? And if they do make both of them, that would be an unprecedented number of six Planeswalkers in a set. And if they did have six Planeswalkers, why stop there? Why not bring back all of them to stop the Eldrazi?
Another thing to keep in mind is that Nissa has been playing around with Black mana, and has teamed up with Sorin Markov. Maybe we will also see her as a Black Green Planeswalker?
As big as the Rise of the Eldrazi set may be, there may not be enough room for extra Planeswalkers. Kozilek, Butcher of Truth has already been introduced, and he is number six out of the set meaning there may be five that come before him, and maybe more after him.
(picture courtesy of mtgsalvation.com)