I think the DLC will be released in the autumn Steam sale. It is foolish not to use such an excuse to increase sales, but according to the rules of Steam, you cannot set a discount earlier than three months from the product release.
The second question that worries me is testing. Updates were usually released raw and tested by enthusiasts for several weeks. But this cannot be done with DLC, since it is paid. How will Klei test DLC? On your own? Open early access to DLC for everyone who bought the game? Selling a pre-order for the DLC and giving them a test? The latter is most logical, but then it would be high time to announce to publish the news about the pre-order sale.
Sorry for google translation.