As annoying as it would be to some, I think the most timely place to set GTA VI would be San Fierro. Unless they change the time period, San Francisco is *the* city of the present moment, for good and for bad. Vice City would be very boring in any other time period than the 1980s. Liberty City has been done recently. A Chicago fascmile has not yet been done, but I'm not sure there is enough depth there for a proper setting. San Fierro, in the very least, has to be one of the top contenders.
I would love to take out some frustration and cause mayhem to fictional tech companies...
For those suggesting it's the whole US I would hope Rockstars knows better than to pursue that. It would never work and they'd lose the polished focus all of the games have had.
The only other location that has as much going on right now, and has enough universal recognition (their entire story arc, satire, etc relies on people essentially understanding the basics and some of the cultural-political-social intricacies of the location) besides NYC is London. But London still isn't as loaded as San Francisco.
San Francisco has three incredibly diverse cities very close to each other. San Francisco is the dense urban metropolis with staggeringly ridiculous headlines every day ($3600/mo avg rent, out of control homeless, techies everywhere, violence everywhere, enormous building boom, Elon Musk, Marc Benioff, Apple, Air BnB, Uber, etc etc), Oakland is the quickly-gentrifying town that is one part hipster, one part gangster, plus it has the blue collar component of the Port of Oakland. And San Jose is this weird ambiguous tech factory. North Bay has all of the wineries and small towns. The central valley is a short drive away. The immediate geography is stunning and arguably more varied than Southern California. The BAY. I honestly can't think of a better location (even counting ones that have already been done) for a GTA game than the bay area. Even the shape of it is conducive to a GTA game. With next gen systems, they could probably get close to a 1:1 representation of it if they chose to.
If GTA is meant to be a strong critique of western culture, San Francisco is the best place to place that critique at the present moment.