Please remember that these are rumors, as no official announcements have yet been made.
The 6th ed Starter Set will be Dark Angels vs Chaos
The Chaos forces are made up of CSM Chosen, as well as Cultists/Traitors and a funky Chaos Dreadnought.... amongst others
I'm expecting the Starter Set mid September
Starter news I have heard from several places now:
Ravenwing bikes (3 likely)
Dreadnought seems likely, as it adds vehicle rules. CSM squad may be referring to the chosen, or may not. Posessed will have to see, but they would add a daemonic aspect to the force.
The Chaos forces
I have seen the Chaos Dread, and been told that it will not be in the Starter Set. the new plastic chaos dread is gorgeous, and has a tonne of options on the sprue. the 6th Edition starter box will not include a Chaos Dread, but there will be a separate plastic kit.
The Dark Angels
I can confirm 100% that the Starter Set is slated for a September release
The starter is due September 1st, staffers have a block for holidays on that weekend and the following weeks.
There will be two versions of the starter set for the first time. Rather than the box containing two armies each will have one, so Dark Angels versions and a Traitor version. Each will have a selection of figures, but the extra space in the box will focus on scenery. So there will be a small Dark Angels force with a Dark Angels specific piece of scenery in one box, and a small Traitor force and Chaos specific scenery in the other.
via Beasts of War
Starter set will include Belial for Deathwing.