So far, in terms of information on Khans of Tarkir, the upcoming fall set, we now know much more than we did a week ago but still very little in terms of concrete evidence. In fact, this is a bottom-up (read: mechanics-came-first, flavor-second) designed block, but we don't even know the mechanical theme it supports.
There has been much conjecture, as there always is, so here's what's likely:
Combat matters. At first blush, this is too broad to be a block theme. After all, combat is always supposed to matter in Magic. However, given the setting, this is only natural - a war-torn steppe of constant battle should give rise to a block that puts extra emphasis on different approaches to war, mixed unit tactics, combat tricks, and the like.
Creature types matter. The first tribal block was Onslaught, which was near-universally beloved (though it's certainly not my favorite, it produced some classic cards, even if the storyline is best forgotten). The second was Lorwyn, in which disastrous design choices were made (but the story was great - funny how that works). Innistrad block had a tribal subtheme, and R&D is now cautious about the linearity of deckbuilding that arises in a tribal environment. Still, it's popular, and enough time has passed.
Wedge block. Okay, so this is a longshot, with a multicolor block still in Standard. But the hues present in the preview image suggest a white-black-red faction, and this could be the thing players have been asking for for a long time that Mark Rosewater promised in this block.