I am hoping 90's, 80's would be cool too... but more these eras from a musical standpoint..
50's would work, not for music, but for greaser gangs an shit... its a good area to work from.
For a place setting, I highly doubt it will be GTA: Chicago, since GTA3 and all the press Rockstar get, they will not name their game after a major city. It would suck to be sued by the city of Chicago.
I also doubt it would be GTA: Carcer City, Manhunt an GTA just Acknowledge each other in the games cause its all Rockstar fam, thats all.
I would like to see ViceCity/SanAndreas/Liberty plus some extra citys all together... as long as there are definite time changes to the city's, I see no problem re-using them... I think re-using past GTA city's adds solidarity to the series.