[Disclosure: Anthony Burch, the writer of Borderlands 2, was previously employed at Destructoid. As always, no relationships, personal or professional, were factored into the review.]
The best downloadable content, in my experience, serves as a low-risk workshop to spitball and prototype new ideas. Sometimes those ideas pop up later in, say, sequels: it’s impossible to get from Dragon Age: Origins to Dragon Age II, for example, without incorporating some of the mechanics first introduced in Dragon Age: Awakenings.
Or sometimes, DLC is a way to meet fan demands, to raise a level cap or introduce a weapons storage system into a loot-driven RPG. It’s a way to explore new areas of a game’s story or universe, or to fill in the gaps of some expository history.
It’s also, I guess, a way to sell horse armor, alternate outfits, and snake oil to unsuspecting consumers.
But the long view of the two Borderlands 2 DLC campaigns to date will show that Captain Scarlett and her Pirate Booty and the more recent Mr. Torgue’s Campaign of Carnage are addressing the fundamental question of what kind of franchise Borderlands wants to be.