LA Noire could've been so much better if they had some kind of focus on the mafia, or something similar to the upcoming film, Gangster Squad.
It's also important to remember that Rockstar draws heavy influence from many films and TV shows. Vice City had a lot of callbacks to Miami Vice and even some easter eggs from Scarface. Also, missions like the Bank Heist in GTA IV had nods to films various gangster and bank heist films. So when you think of movies made or based in the 70's, a big one that comes up is The Godfather. Now there's already made a game based on that film, and Mafia II also dealth with the team of the post world war mob, so I'm not sure if they would focus on that, but they could also add in some disco, a la Saturday Night Fever. Possibly even go the route of Taxi Driver, although that was sort of touched on in LA Noire.
I think the 70's were definitely one of the most influential decades on American Pop culture, so it seems like a right fit for Rockstar to go to next.
It's more of a question as to whether that game will ever come out at this point.