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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3-2 = 1 one copy of Half Life 3 Half life is a game games are fun fun has 3 letters oh shit.. [spoiler]the Illuminati invented fun![/spoiler]