In the current era of Magic design, much is being made of the distinction between top-down (flavor-driven) and bottom-up (mechanics-driven) blocks. Innistrad is held up as the preeminent example of the former, and Ravnica and Return to Ravnica as the latter. Naturally, everyone is curious as to which type the coming block will be, and although Mark Rosewater has previously stated it is bottom-up, he answered a Tumblr question on his blog and gave some new information recently.
When lemidget42 asked, "Can you tell us if Huey/Dewey/Louie is a thematic block or a mechanic block? Like, Innistrad/Theros was thematic while Ravnica was built around the mechanic of two-color," Rosewater replied:
I can only restate what I’ve already said because it’s too early to talk about Huey block. It’s a new plane, it’s bottom-up design and there is something special about the block structure.
I believe you all are really going to like it so I am quite eager to talk about Huey block, but you all will have to wait a little longer.
There's only one piece of new information here, and it does serve to confirm the suspicions of most people, but it's a doozie: Huey block is not "Return to Zendikar" or "Innistrad 2," as some had hoped for; it is an entirely new plane. The current Magic rumor community presumption is that it's Khanar - or Tarkir - a war-fraught Mongolian-inspired plane that may well be the long-fabled "combat matters" block.