Tyranids have been rumored to be released after Space Marines probably around the first of 2014 or if we are lucky the end of this year. This is one of the codices I am looking forward to the most, as it will alter and change a lot that is going on in regards to allies for the army. This rumor gets into what may be going on.
Please remember that these are rumors. This is a ways out, so please add in a little extra salt on this one.
This does make sense though and is logical. If a codex is coming out, of course it would be awesome if a codex supplement followed it, as that may be an incoming trend of future codex releases.
Here is the rumor,
via an anonymous source
I've heard about a supplement for tyranids, that they will be getting a genestealer cult supplement possibly something akin the recent GW tourny rules we saw at Warhammer World.