Each of the last three blocks of Magic have, to some degree, been "top-down" - that is, designed with story and flavor in mind first, and then mechanics interwoven in a holistic way to support that story. Innistrad was Gothic Horror with a religious twist ending in which the archangel Avacyn returned to cleanse the world, Return to Ravnica had established flavor of a city plane and ten guilds to work with, and now Theros is Magic's riff on Greek mythology. This isn't a neverending trend, though, according to Mark Rosewater. A couple of months ago, when Tumblr user ascoat asked him on his question-and-answer blog, "If RtR was a throwback to an earlier age of design, what would a bottom-up block look like in the current age of design? Or are we going to have to wait and see? : - )" he answered:
Huey (the 2014 fall large set) will answer that for you.
Of course, this doesn't mean much yet; but it is an indication that Huey/Dewey/Louie block will be a more typical "Theme N matters" block like the ones that arose in the previous age of design. Many players were disappointed with the fact that Theros wasn't truly an "enchantment matters" block, and instead simply tried to use enchantments to tell a story, thus somewhat depriving the card type of its time to shine. Those players will certainly appreciate the news that the 2014-2015 block will have a tighter mechanical focus.
In addition, Rosewater revealed a block he'd like to design in his 20 Years of Magic interview with Marshall Sutcliffe at this year's World Magic Cup. He said that though Scourge, the final set of Onslaught block, was supposed to be "the Dragon set," that didn't come through very well (it mostly had a high-converted-mana-cost-matters theme and some minor Dragon tribal themes) and he believes an entire block could be pinned on Dragons someday. Now, the Dragon type is easily the most popular creature type in the game, according to Rosewater, and it's not only red's iconic rare and mythic creature type but has resonance in every other color as well. It'd be a challenge, of course, to justify dragons in colors like white and green en masse and to fit the theme at lower rarities, but I'd be surprised if we didn't see a big, splashy Dragon Block someday.