Dying Light is getting an expansion and...that's pretty much all we actually know about it at this point. Details are very, very thin on the ground, but the name and screenshots suggest it takes place after the main game, possibly in a new location, and will have vehicles. It's also possible that it may be standalone, though the wording around that is open to interpretation as Techland have said it will be "available" separately, but also included as part of the Season Pass. That more likely means it will something you can purchase outside of the Season Pass, but one can hope right?
We do not, at present, have a verified price or release date either. But judging from the amount of effort Techland have already put into the game, we can expect it to be a solid piece of content and well worth your time and money. So keep your eyes peeled folks, because the light has not died yet.