There's an online rumor, NOT official GTA news, from a Rockstar insider named Matheusvictorbr, who leaked some information for the future GTA 6 game. Remember this is all rumors right now and nothing has been confirmed by Rockstar.
The GTA 6 rumor
So here we go. The GTA 6 story will have two main characters, twin siblings (one male and the other female). The siblings will go their own ways after the death of their parents by a cartel in 2003. The story will then go ahead to the 2020s and the two siblings will meet once again but they'll be on different sides. The male sibling became a DOA agent and the female sibling joined the cartel and is working her way up, waiting to seek revenge her own way. The game will take place in Carcer City, Cuba, Columbia, and Vice City (from GTA: Vice City). The game will release in 2024.
All of this information is from the "EXCLUSIVE: GTA 6 story details, release date & new GTA Online map" article on xfire.com.
What do I think?
My own opinion about this rumor regarding the single-player story in GTA 6 is that I find this story interesting and it definitely sounds like it would fit in the world of GTA. I'm hoping we hear something about GTA 6 this year but I'm not going to hold my breath for that. I also don't expect GTA 6 to come out in 2024 as the rumor suggests, I expect a later date. I do wonder if we'll be able to switch characters back and forth as in GTA V, or will we have to pick a character at the start and only play them throughout the story, or will it go back-and-forth between characters at certain parts of the story.