I am of the opinion, ESPECIALLY IN RPG's, that if the player has exhausted what the game has to offer, then it is time for user-generated content to become relevant and for players to exhaust other player's content, which is pretty much infinite if the game is as big as the Elder Scrolls series or the Half-Life series, among others. Another of my opinions is that if the story is constructed as non-linear fragments, the possibilities become endless for expansions, even micro-expansions, and are not limited by a linear continuous story.
EDIT: Another exciting opportunity made possible only recently with newer processors is real-time procedural generation of quality content. I have seen procedurally generated graphics that are stunning. There is a blog by programmer Miguel Cepero called Procedural World that involves his server farm generating amazing structures and terrains on-the-fly. There is already a game called Anteworld made with a custom engine called Outerra that simulates the entire Earth using procedural fractal algorithms that are based on Earth's continents.