The next logical step for GTA is to have the game centered around an organized crime family.
There are several options to choose from: Italian, Russian, Triad, Yakuza, Cartel etc.
-This now would operate as a Tycoon type of game after the story is completed. You are now left with an organization to run with several businesses and employees. The decisions you make affects the environment around you. And players will have different goals to reach. Think of it as a sports franchise mode for the GTA series. Imagine Zynga's Mafia Wars in full playable 3d. It also solves the problem for the people who aren't into sharing their world online.
-This also allows for players to feel like they are apart of something within the game.You can have that feel you had in GTA San Andreas when you could always ask Grove Street members to have your back and work for you. It gives a sense of status within a community. I prefer this instead of feeling like a random guy with no where to go when story mode is complete.
Imagine a long story mode where you actually play multiple time periods of an Italian mobsters life. From becoming notorious in your neighborhood for violence and loan sharking. Hanging with your crew in an infamous social club in the golden era of the mob. Becoming a top earner and made man by committing hits, heist, loan sharking, gambling rings, boiler rooms, scams, drug dealing and more. All at your own free will. Then eventually becoming The Don and running whats left of the current mob.
I've been waiting for this for a long time and I feel this is the natural evolution from 3 protagonist. Rockstar could allow the possibility to switch between members of your crew for a variety of experience.