Really really hope that the Wedge theme in the first set of Tarkir means we'll be getting the wedge tri-land taplands to finish the shard cycle from alara of
Seaside Citadel and company.
Might be interesting if they lowered the rarity to common and reprinted the shard cycle at common in a later set of the block, for the effects it would have on pauper and block draft, as well as the standards it would set for tap-land based color-fixing. Although keeping them at uncommon would be fine too, I'd leave that up to WotC internal playtesting.
Also hope they continue to take lessons they seem to have learned in Theros block and M15 about auras, and ensure they have built in ways to try to make up for the '2-for-1' threat, like giving them decent ETB effects, cantriping, self-bounce, or making the target removal resistant, or leaving creatures behind, etc. so we don't return to largely unplayable auras as a standard.
I'm kinda hoping for a WUBRG colored dragon (may be more than 5 cmc) in one of the sets that has all the mechanics of the 5 warbands, to represent the 'ultimate' ideals of dragonkind espoused by the first set in one of the later sets, so long as those mechanics seem to be applicable to a creature like a dragon. Or at least one with hints of the 5 virtues of dragons that the warbands are concerned with. Not sure if I'd want it legendary or not, some of me wants it legendary so it is commander applicable, but some of me doesn't so that it represents simply a potent kind of base dragon hinting that all dragons have this kind of potential and this is one of the strongest dragon breeds. Both would be really awesome, perhaps one showing the virtues another way, while one actually uses the warband mechanics. Maybe the one with the virtues shown another way could be a non-legendary in a set taking place in the past of that world, while the legendary one would have the warband mechanics and show up in a set that is in the present again with dragons returned. That would probably fulfill more than the WUBRG quota for the block, providing a new WUBRG legend for Commander and a return to the mechanics from the wedge set without actually having a wedge theme in a second set, to help tie sets together and reiterate themes.
Also hope the rakshasa looking things from the art are partly blue cats with some shapeshifting related abilities, I'd love having a blue (or at least partly blue) cat race, and rakshasa in D&D style stuff seem to fit that theme, what with their shapeshifting and mindreading abilities.
Also hope the orcs are cool and interesting as a tribe in this block.