M15

There is a lot of expeculation about the planeswalkers on M15 core set, so far we know that Nissa could be the green planeswalker. It could be the same Nissa or a new one. 

Garruk is the other surprise. We can especulate that he is going to be mono black, because Funko and Magic the Gathering are going to sell Pop Vinyl. With the launch of M15

Since Nicol Bolas these is the second set that could have 6 planeswalkers and maybe Garruk could be multicolor. But so far all is uncertain.