Warning: If you have agreed to the NDA and know any information; it is ILLEGAL to post it here and you will be fined or even worse.
Now to the point of the post for the rest of who have not accepted an NDA and just want to have fun guessing!
I personally think they are testing Windows Sever 2016 in the NDA environment. I have heard a lot of staff saying on reddit that we would get Windoows Server 2016 very soon. That is my guess; so what is yours?