A marketing story often works best with over-delivery. Over-delivery means that the way that the game extends has never been seen before by the intended audience.
Over-delivery can mean delivery on content or on game dynamics, or some other factor like exceptionally funny writing. Angry Birds over-delivers on level content. Blizzard have long allowed players to play multiplayer Starcraft across Battle.net for free, and Valve do similar with their Counter Strike and Left 4 Dead games.
Over-delivery has to feel unique for your audience. Copying someone else’s over-delivery won’t do, nor will over-delivery that’s about the developer and not the player. And tying over-delivery to money (charging for it) is not over-delivery at all. It’s just delivery.