War is hell, and from what we know of Tarkir, this plane is a world of constant war. It is also the homeworld of the planeswalker Sarkhan, and the setting of the block that is to kick off with this year's fall set. Now, to get across the fact that Tarkir is a scary and brutal place - not beautiful and perilous like Theros or moody and creepy like Innistrad - R&D will make some considered design choices. Here are some likely possibilities for Tarkir's most unpleasant themes:
-1/-1 counters. It's been a while since the last non-+1/+1 counter block (Scars of Mirrodin came out in fall 2010), and since the two principal types of creature counter can't coexist in one block to prevent confusion, -1/-1 counters have to have the spotlight back eventually. All the better on Tarkir, to showcase the grievous suffering that takes place there.
All the Dragons are undead. One informal design rule is that every block has Dragons, simply because they're the best-liked by far of the iconic creature types. Even the Gothic world of Innistrad has them, even though that setting is the last place a Dragon would traditionally be written into. But canonically, the Tarkir warlords have hunted Dragons to extinction (to showcase how hardcore they are), which means that in order to get them in card form, R&D will have to design zombified or skeletal ones.
A recurring villain recurs. The characters and factions meant to be Magic's repeat antagonists have been pretty camera-shy, with neither Nicol Bolas nor the Eldrazi nor the Phyrexians having been seen since their establishing or re-establishing appearances. However, with Sarkhan as a former agent of Bolas and no other living Dragons to challenge him, the devious ancient planeswalker might finally show up again,