It's what we like to see: The tease quickly followed by an official announcement.
It's called The Following and it's described as a "feature-heavy, story-based expansion pack" that is apparently quite large. Said Producer Tymon Smektala:
"For Dying Light: The Following we’re adding a number of bold game-changers to create one massive expansion. Something big that will give the game a brand-new flavor. The new map alone is the same size as all the previous maps from Dying Light combined, so there is a lot we’re packing in here.
We also kept a close eye on player feedback to help tailor what we’ve created here. Hopefully people will see that this new expansion is first and foremost, for the fans."
The Following will be available to purchase separately, but will be free to those who snagged the Dying Light Season Pass. There's no date or price just yet but Techland said we'll get the full reveal next month at Gamescom.
Related Game(s): Dying Light