I'd really like to see a GTA set in 2004 - 2006 or so New Orleans. Or a modern day "Crescent City" that just happens to be affected by a massive hurricane and flooding mid-game.
GTA4 addressed post-9/11 terrorism stuff.
GTA5 addressed the housing crash and recession.
Seems like race would be a major topic today and a natural thing for R* to try and address. Pre- and post-Katrina New Orleans would be perfect for that. The "looters" photos, Danziger, etc. Really interesting area with some nice terrain variety (I'd love to see R*'s take on the bayou). Heck, even True Detective it up a bit and give us a cult in the swamp. =)
Failing that, if we're visiting the past, I'd love to see 70's San Francisco or New York (though I don't really want to go back to Liberty City so soon).