The way to keep interest in the late game is to start a new game. Seriously, all games of this type suffer from the same problem. They are more or less compelling at the outset (this one is extremely compelling), when success feels like it is in question. So it is highly engaging to pay attention to the details of strategy. But once success is assured, it becomes more like a job. Some people solve this by setting up their own side goals. Me, as soon as I start thinking to myself that it doesn't really matter what I build next... I save the game "just in case" and go get a cup of coffee so as to mull over my choice of next civ and sit down hit the button to create a new game -- which I can count on to be riveting and not at all a job for hours to come. Repeat after me: "I do not need to see a victory screen."