Rockstar will bring more missions to GTA Online so that players can play on their own; although many wished for it to not be the GTA 5 DLC. It's been more than seven years since the first release of Grand Theft Auto 5, so it seems safe to say that there won't be a new DLC story for it, despite the early hopes.
It does not say what form this new content may take, but if all of this is intended to be an oblique reference to Grand Theft Auto 6, it is not made clear enough and it really seems as if more GTA Online content is just being discussed.
“We’re going to inject more of that single-player element in there” - GTA design director, Scott Butchard.