Ever since the name of Khans of Tarkir was confirmed for our upcoming fall large set, much has been made of the structure of it. More often than not, you see, sets that take place on never-before-seen planes are named just after that world (See: Mirrodin, Lorwyn, Shadowmoor, Zendikar, Innistrad, Theros). The exceptions all somehow draw attention to an important story and/or mechanical feature: Shards of Alara isn't called just Alara to emphasize that the plane isn't even one world anymore, and to make the fact that the Shards reunited in Alara Reborn all the more special, and Champions/Betrayers/Saviors of Kamigawa all focus on a particular group of (legendary) characters who indeed shape the fate of that plane.
So since Khans of Tarkir is a departure from the norm, and previous departures from the norm have been for special reasons, it follows that, well, the "Khans" are significant. Mark Rosewater confirmed this on his Tumblr blog, when doopboopdoop asked, "What was the decision to name it Khans of Tarkir instead of just Tarkir like most previous blocks having the first set just be the worlds name (Theros, Innistrad, Zendikar etc.)" and he responded:
All will be explained. Not now though.
Now, if the answer were "just for flair/it sounded cool," Mark wouldn't be so dramatic about it - he doesn't lie, and doesn't outright imply things that aren't true. So the Khans themselves are specific important individuals on Tarkir - we're looking at a cycle of legendary creatures, perhaps, or, as the prevailing theory goes, the Khans lead five three-color wedge factions that form either a major or minor theme in order to make this the first ever "wedge block."