Rockstar posted an Instagram story with an old photo of the young basketball player LeBron James with a copy of GTA: Vice City in front of him. This is their official account, not some pranker. It might be a serious clue. They chose GTA: Vice City for a reason.
It is worth mentioning that in 2010-2014 LeBron played for the "Miami Heat" team wearing the number 6. A hint of GTA 6?