The protagonist of the game is under question as well, and Rockstar apparently have various ideas for that which they want to try out. The idea introduced in “GTA 5” to have several protagonists might be continued in the next game as well, while there have also been rumors that the next game might feature a female protagonist for the first time.
It’s not known if those rumors are related though – in other words, it’s possible that the next “GTA game has a female protagonist as part of a multi-character cast, which would make it an amazing entry into the series if it’s pulled off right. That could open up some great options for the plot, among other things – and the female character could also add a different edge to the game.
If the 2018-2020 release window is real, then we will probably start hearing more about the game in 2016-2017, but getting any sort of official information before that seems very unlikely, especially knowing Rockstar and how secretive they like to be about their upcoming projects.