Or a single player game that requires periodic online cooperation to complete.
Think of a mission that requires a bank robbery. Instead of just putting together a crew like you did in GTA5 you join a crew online of random players and hit the bank. The level of success determines the take in your single player game.
Or there is a campaign that is structured like a single player game but is combined online scenarios. You log in, join a crew to complete this mission, join a different crew for that mission, join yet another crew on a different day to do the third mission. You get the idea.