There's been talk for a long time now about GTA 5's single-player DLC and plans for the character of Trevor to be involved in a big way - but the belief is that it was cancelled in favour of focusing on GTA Online instead.
That's essentially what was reiterated by Joe Robino, camera artist and virtual cinematographer on GTA 5, in an interview with the SanInPlay YouTube channel earlier this week - where he revealed that he worked on the planned DLC:
"A lot of the team went to do Red Dead Redemption 2 right away and I took on this other project that was a standalone DLC for GTA that never came out and it was kick ass."
"It was really, really good. But when GTA Online came out it was so much of a cash cow and people were loving it so much that it was hard to make an argument that a standalone DLC would outcompete that. I think looking back now, I would say that you could probably do both. But that was a business decision they made. And I was a little upset about that."
According to Robino, Steven Ogg [Trevor's actor] was indeed heavily involved in the DLC, and the team spent "so much money" creating it. When it was scrapped though, a lot of the content was seemingly recycled for GTA Online.
Nevertheless, he says he was a "little sour" about the decision to cancel the DLC at the time, believing it to be "awesome" and worthy of finishing. He ultimately ended up leaving Rockstar Games in 2016 and going on to work for Ubisoft (Assassin's Creed Odyssey) and then Xbox Game Studios (Forza Motorsport, The Outer Worlds 2).