The date 1978 pops up twice in the history of the Five Families, they being the Ancelotti's, the Pavano's, the Lupisella's, the Gambetti's, and the Messina's. Jon Gravelli (the old dying mobster that Niko does work for) became Don of the Gambetti's after the Don's death in 1978. Giovanni Ancelotti became Don of his family after his uncle's death in '78 as well. There was also a "bloody power struggle" in the Messina Family, though it is not stated when, a love triangle in the Pavano Family, a mysterious old Don in the Lupisella Family, and more strange facts. It's probable that more will be known of the families in the future, especially after "the Ballad of Gay Tony" is released (as it concerns the Ancelotti Family and their struggles). But whether the next GTA will be in '78 is unknown. Also, if the next game will take place in this time period, a good city for it to be set would be a Detroit-based city, as it was going through major struggles with crime, riots, race, and more during the 60's and 70's, and it still is to an extent.
yer, that would be cool to see liberty city in the 70s. and thats what rockstar is probably thinking of because look at all the gta 4 era games there all based in liberty city and it would probably stay like that for the rest of the era.