After Time Spiral block and the introduction of various planeswalkers as both cards and recurring storyline characters, there began a concerted effort in Magic's Creative department to have an overarching story from block to block, even between different planes. Part of this came in the form of antagonists set out to be recurring villains, though each of the so-called Big Bads of Magic have only appeared in a mainline block once: Nicol Bolas in Shards of Alara, the Eldrazi in Zendikar, and the Phyrexians in Scars of Mirrodin.
Post-Scars, however, we have seen three blocks with either an alternate villain (Xenagos in Theros) or no single proper villain at all (Return to Ravnica and arguably Innistrad - Griselbrand's real villainy happened offstage and long before the block began when he trapped Avacyn alongside himself). So if these characters are recurring villains, when are they going to recur?
Such a question was on Tumblr user bloody-altima's mind when they asked Mark Rosewater on his blog, "So in the past, you've said that the three biggest Big Bads in Magic are Nicol Bolas, last seen directly in Alara block and indirectly in Mirodin and Zendikar, Phyrexia, last seen on Mirrodin, and the Eldrazi, last seen enjoying a delicious Zendikar smoothy. The most recent of these was two (sic; New Phyrexia was released in 2011 - Ed.) years ago. Will we be getting a block where our biggest villains actually do something soon?" Rosewater's response was simply "Patience."
This doesn't indicate a time-frame, of course, but many are interpreting this to mean one of the three main antagonists will reappear in the Fall 2014 large set and the ensuing block. Meanwhile, others see Theros as the start of a villainous career for Xenagos, who, as someone who's still a planeswalker despite also having become divine, presumably could leave the plane after having his godhood taken away by our heroes in Journey into Nyx.