So who know whats going on for sure.
I can tell you though, that GW loves to release Xenos races late in the year, with every fall/winter for the last five years seeing something Xenos-ish around then.
Here's the latest hotlinks form the tubes -MBG
Tyranid Allies? http://natfka.blogspot.com/2013/08/tyranids-can-ally-with-themselves.html
Tyranid Tidbits http://apocalypse40k.blogspot.com/2013/08/new-tyranid-rumors.html