Every so often, the Magic rumor community is able to find a likely-looking trademark that Wizards of the Coast has registered for an upcoming set, and no one is on the case more often than MTGSalvation's fnord. This time around, fnord has uncovered the usual application for the name Khans of Tarkir, which sounds very thematically similar to Warlords of Khanar, which was applied for last year.
Ordinarily this would be the time when the trademarked name would be more or less unofficially confirmed as the winter or spring 2015 expansion. However, the wrinkle in the idea this time is that the name Khans of Tarkir is too similar to Warlords of Khanar. They both seem to be introducing a plane for the first time; "Khan" and "Warlord" are nearly synonyms; and having a set referring to Khans in the same block that takes place on Khanar would be linguistically awkward, to say the least.
This all raises the serious possibility that the application for Warlords of Khanar was rejected, and Wizards of the Coast instead chose to submit an entirely new entry. Theros, after all, underwent the same fate when it was first applied for, but the rejection was contested and it of course became the first set of the current block. Perhaps events took a different course this time.
Either way, it seems we're destined to have a Mongolian-themed block next fall. Mark Rosewater, however, says the design was mechanical-based - bottom-up - and this may be something like the "combat matters" block.