In a discussion on whether or not the CSM rumors were really going to happen, Atia on Bolter and Chainsword mentioned what is coming out later this month... Stormcast Eternals, Knight Heraldor and Knight Vexillor.
Please remember that at this point these are still rumors, and a rumored chaos marine codex is still just that as well, a rumor. Age of Sigmar is being said to turn over the reigns to 40k soon, with a Tau codex in the future. When this will be and whether or not a chaos marine codex is on the horizon, is still to be seen. I have not personally had any solid leads one way or the other.. except that Tau are indeed coming soon (just no dates)
via Atia on Bolter and Chainsword
September 19th is Stormcast Eternals afaik ... Knight Heraldor and Knight Vexillor, no Tau, no Chaos, no Unicorns
