Speculation: Zendikar Confirmed

[Pi]

Overlord Hasbro's greatest motivation is profit. The Zendikar block is still being sold, as people hunt for eldrazi titans and fetchlands.
There is also a chronological "aligning of the stars". Mirrodin was printed in 2003. Ravnica was printed in 2005. Scars of mirrodin in 2010, RTR in 2012. 7 years between reprinting blocks. Zendikar printed in 2009, should have a reprint in 2016.
Also the lore is circling back onto itself. Ugin, the spirit dragon, who sealed away the eldrazi on Zendikar, was found dead on his home plane of Tarkir. {Sorin Markov}, another one of the 3 planeswalkers, to help seal the eldrazi, has come to Tarkir looking for his help. The third planeswalker, {Nahiri, the lithomancer}, finally has a card.
Also, the eldrazi are the last large inter-planar issue, not to be revisited. The Phyrexians had their  New Phyrexia, and Bolas has gotten a lot of hype in the code sets, with a focus in m13. 

[Agrus Kos, Enforcer of Truth]

Very good points. One more: a movie based on the superfriends and their battle against the Eldrazi would be amazing, especially if it were paired with a set in which the story played out. WoTC could add millions of new players if they can pull something like that off.

[Dudecore]

Khans is the best selling set of all time. If they wanted to just make money then logic would dictate they stay on Tarkir.

[Pi]

Very good points. One more: a movie based on the superfriends and their battle against the Eldrazi would be amazing, especially if it were paired with a set in which the story played out. WoTC could add millions of new players if they can pull something like that off.

Vorthos following the lore would be the equivalent of "reading the book" before seeing the movie. I knew all of this useless information would come in handy some day.

[Agrus Kos, Enforcer of Truth]

Khans is the best selling set of all time. If they wanted to just make money then logic would dictate they stay on Tarkir.

Khans being the best selling set of all time has nothing to do with Tarkir. The last four fall sets have been the best selling sets of all time, it is simply because of the growth of the game. Khans gets an extra boost because of the fetches, but the plane of Tarkir is by no means the most popular plane, and even WotC knows that.

[Dudecore]

Well I mean...we're taking about making money. Khans of Tarkir makes plenty of money...the most ever. We will return to Zendikar because of the open story and it was popular, not because of money.

[Pi]

Well I mean...we're taking about making money. Khans of Tarkir makes plenty of money...the most ever. We will return to Zendikar because of the open story and it was popular, not because of money.

I phrased that incorrectly. Zendikar scored well in their market ratings, which is how things get reprinted. The constant expansion of the game, makes the newest set the "best money maker". Evidence from 2010: http://archive.wizards.com/Magic/magazine/article.aspx?x=mtg/daily/mm/110. I was not able to find an article from wizards, that explained that because Lorwyn/Shadowmoor "scored badly" it would not be reprinted. The actual profit is not what determines reprintability, it is their market testing.

[Pi]

http://magic.wizards.com/en/articles/archive/feature/whats-story-magic-origins-2015-02-27
tl;dr: Magic Origins is going to visit the home worlds of the 5 walkers; Nissa, Gideon, Liliana, Chandra, and Jace

Nissa's home plane is Zendikar, therefore the theory is still valid.

[Aladormax]

Zendikar was confirmed though in that article

[Munchlax]

But after they return to zendikar.... what is the next big threat? O.O  what are we going to live for? What is the purpose of anything?

Isn't evil garruk still running around?

[Estel2931]

But after they return to zendikar.... what is the next big threat? O.O  what are we going to live for? What is the purpose of anything?

Phyrexians...always gotta worry about them lol

[lotrwk]

Well I mean...we're taking about making money. Khans of Tarkir makes plenty of money...the most ever. We will return to Zendikar because of the open story and it was popular, not because of money.

Then why not stay on theros? That was, at one point, the most sold set. Ever. But it's not now. With magics growth, chances are every first set of a new block will be the best seller.

[LinkCelestrial]

I put all these peices together a while ago. Back when Khans was just coming out. I do suspect the next arc will be Garruk. I'm just wondering how many people he gets to kill before he gets dealt with.

[DirtyMustachio]

Fetches sold khans not the mechanics

[Pi]

I put all these peices together a while ago. Back when Khans was just coming out. I do suspect the next arc will be Garruk. I'm just wondering how many people he gets to kill before he gets dealt with.

And my original post, was when Khans came out. And now return to Zendikar is confirmed, by wizards.