Story DLC is not Part of Rockstar's Plans for GTA V let it die people.
So basically we should just forget about it and let it die. While GTA Online has seen itself receiving a slew of updates since its launch, Grand Theft Auto V and its single-player story has had nothing, despite mention of "substantial" story DLC touted for 2014. Clearly, that didn't materialise and that's not set to change, as Rockstar has stated it's simply not on the cards, as the team has completely shifted its focus to providing content for GTA Online.
So we should ignore the "Right Now" in the sentence?
All I know is...when the game came out on PS3 everyone (including myself) but a select few said, "No way are they making this for "next gen". And those that believed it would had far less confirmation of that happening than what we have for SP DLC. Rockstar is fully capable of walking and chewing bubblegum at the same time. Which is to say...they can work on many projects at once regardless of prioritizing one over another.
They've said they're working on it. One of the main actor's has said they're back in the studio. So until Rockstar unequivocally states..."No SP DLC, I'm just going to assume it's being worked on.