The new DLC "Atomic Ambitions" for Millennia is now available.
It adds an atomic age, as an alternative age similar to the Fallout settings, and also a wasteland age, which is a postapocalyptic age.
Oh, and there are nukes.
The DLC is currently available for 19.99 on Steam.
Paradox recently also released another dev diary about this DLC, please see here.