I hear that there will be story line DLC. What in the world can it be about? Joel has already stopped the Fireflies from trying to create the cure so to me there is no more story to be told concerning Joel and Ellie. It must deal with Joel and Tess before the start of this game. Or maybe dealing with Joel and his brother right after Joel's daughter died.
I think they said if there was a sequel then it wouldn't concern Joel and Ellie. I know the DLC isn't a sequel but may give some insight into how they may approach it. So it may well be an epilogue to them as characters, a side story concerning characters we met or mini-story with all new characters or any of your suggestions. My money is on an epilogue, Joel and Tommy or Joel and Tess, maybe even Henry and Sam surviving before meeting Joel and Ellie?
Also, yeah, may want to pop a spoiler warning in the title.