Although we have little information about the coming sets in Theros block so far, and spoilers won't start until around Christmas the earliest, sometimes we can determine flavor elements from upcoming sets when Magic artists post art pieces that are not covered by non-disclosure agreements on their own websites or sites like DeviantArt. Clint Cearley is one such artist who's illustrated more than thirty Magic cards, and recently, on his portfolio website, he added a classical-themed art piece entitled "The Naiads," which wouldn't look out of place in Theros block.
The artwork depicts five beautiful pale-skinned women, apparently supernatural, reclining in a pond and gazing at the viewer. The central figure, unlike her dark-haired "sisters," has blonde hair and completely black eyes, and has an anaconda winding around her. Since there is no Nyx starfield visible in the art, these strange women cannot be Naiads (a type of divinely-created Nymph) in the Theros sense, but if not all Therosian Sirens have wings, they might well be that type.
All in all, the artwork reads as green-blue in tone - if this is a Theros block card art, the individuals depicted might be followers of Kruphix, the god of time, and the center figure might actually be a legendary creature.