I feel that there's a pretty good chance of a GTA 6 teaser. Their earnings call is on the 20th, and while they won't announce it there, they could hint at something 'from an established franchise', etc.
Based on GTA5, I don't think it'll be released for another two years. This would be early, but settled, in the lifespan of the next gen consoles. GTA5 was an Xbox 360/PS3 game that got an enhanced edition on Xbone/PS4 & PC, keep in mind. Going straight to next gen entirely might make more sense. They probably made more money from releasing on two console generations, but all the versions needed a lot of ongoing resources to make and maintain, and Rockstar is trying to be kinder to their employees.
As for speculating a location, it's Vice City's (Miami) turn.
I think that it would be Vice City again. GTA: Vice City came out in 2002, so a much longer stretch than the 2001/2008 iterations of Liberty City, where fans sort of complained about them being so quick to reuse a location, although it turned out fine. Two more years of development would put it in 2022, which is VC's 20th anniversary, unbelievably.
But are there any other cities to base GTA locations on other than NY, Miami, LA/CA & Las Vegas? Where are the biggest American cities with the most diverse and dynamic things happening? Cities unlike any other? Chicago is big, but why not just do NY? Texas, why not just do Vegas?, etc.