Supposedly, Rockstar Games is planning to shift its focus on single-player DLCs for Grand Theft Auto 6, as per a reliable insider.
The concept of DLC isn't new to the Grand Theft Auto series, as we've seen with the very successful DLC released for GTA 4. Grand Theft Auto 4 received an expanded experience with The Lost and Damned and The Ballad of Gay Tony single-player DLCs. The DLCs featured new playable characters, brand new stories, and gameplay features.

Fans expected Grand Theft Auto 5 to get the same love for single-player, but it never came to light, albeit it was apparently planned by Rockstar Games. Yes, Rockstar Games did intend on introducing post-launch single-player content for the story mode beyond heists, but GTA Online took off and became incredibly popular, so the studio focused on that instead.
GTA 5 was originally supposed to receive a substantial amount of post-launch content, which was scrapped in favor of the highly sought-after GTA Online.