This info was just leaked on the twitterverse.
I was honestly really hoping for a copy of Fallout 4. At least that will be one of the few exotics no one has Xurned still.
EDIT: Lord Malak has reminded me that Xur will also be selling the Illuminati engram.
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X U R = 3 letters. H A L F = 4 letters. L I F E = 4 letters. 3 = 3. 3+4+4+3=14 T W I T T E R V E R S E = 12 letters. 14+12=26 L O R D M A L A K = 9 letters. I L L U M I N A T I = 10 letters. 9+10=19 26-19=7 E N G R A M = 6 letters. 7-6=1 1 all seeing eye of osiris. Confirm'd destiny xur is illoominatry confirmed by trials of osiris all seeing eye triangles. ANDY2SPOOKY5HALFLIFEXUR3