Do not think of them as a typical add-on like cod or battle field 4. Treat them or look at them as if they are expansions similar to World of Warcraft. They will add new mission, strikes pvp maps, armor and weapons, as well as a new raid. These will not be available at launch, they will be released over the course of the year and then 2 more will be released in the year leading up to the next destiny story line release. This is going to be a 3-4 game story with multiple expansions being released between each game release over the next ten years. These will also add to the story line leading up to the next game.