A release date has been announced by Paradox Interactive for the upcoming Stellaris story pack called Leviathans. Leviathans will be released for PC on October 20th 2016 for $9.99 and will contain a plethora of new content such as Enclaves, Guardians, and the "War in Heaven".
In addition to the release of Leviathans, there will be a free update called "Heinlein". Some of the updates for all players include:
A convenient Auto-Explore feature.
Rally points for your freshly-assembled Navy.
The Expansion Planner Interface.
An overhaul of Strategic Resources.
Clearer specialization/role for ship types.
More options for customizing new galaxies.
New smarter Sector Governors.