I'm still holding out for London, even if Dan Houser said it wasn't very likely. Rockstar also said that Chinatown Wars would be exclusive to the PSP and EFLC would only be on the Xbox 360: Things change, or companies lie in order to make money, whichever you want to look at it. Doesn't mean that London will never happen. Just the chances for now have been drastically reduced.
If I had to pick from that list though, I'd probably go for Philadelphia. I've already suggested the idea of a GTA game set in 1980s Philadelphia during the Philadelphia Mafia War. If you expanded it to include Atlantic City you could include gambling and you'd have that nice, 1980s beach aesthetic that people loved about Vice City, combined with the grim setting of a East Coast city. I think it would be perfect!
As for places not on that list: Somewhere along the U.S./Mexico border would be good if the cartels featured heavily, the cartels are a whole different level compared to the street gangs we're used to in GTA, and I think after playing GTA V it would be a perfect follow-up. Where exactly along the border is still up for debate, but I think El Paso/Ciudad Juarez is top of my list, because you could lampoon the Texas culture, which is pretty much the epitome of America for people who live outside the U.S.
Other suggestions would be Las Venturas in 1977 ( ); or New Orleans maybe just after/before Hurricane Katrina.