This year's ambitious stab at a new format with the summer supplemental product, Conspiracy, has been getting a lot of attention from the Magic rumor community lately, especially after the full reveal of Journey into Nyx, as it's the most concrete partially-spoiled unreleased set we have right now. To be fair, there are a lot of things to be intrigued about - one of which is the fact that, unlike any previous supplemental product, Conspiracy takes place on a specific plane and has some semblance of a storyline, or at least a background.
But so far everyone at R&D has been mum as to what plane we're looking at. Due to the profusion of certain elements - Gothic architecture, ominous angels, ghosts, and three-cornered hats - Innistrad was a fairly popular guess for many rumormongers, despite the lack of anything to definitively identify it as the spooky plane of Avacyn (such as the collar symbol or Vampires in Markov attire, for example). But they should look for a second guess, now - since Mark Rosewater has shot that theory down, officially denying to brian3366 in his "one Conspiracy question for the week" on his Tumblr blog that it's Innistrad.
This means the current leading theory is Dack Fayden's home plane, Fiora, which by all appearances emulates squabbling Renaissance Italian city-states. Since Dack gets his first planeswalker card in this set, it's only a natural fit.