If there's one thing I love, it's studios who develop fully fleshed out DLC packs after the game enters the market. In an age of on-disc day one DLC, thankfully, Bethesda is doing things right with Skyrim.
Of the course of an interview with I Eat Games, lead Skyrim artist Matt Carofano explains that Dawnguard was a result of a brainstorming session after Game Jam. Someone made a Vampire Lord scenario, and the team thought it would be cool enough to fully produce -- hence, a $20 expansion was born.
Of course, there's probably more DLC on the way, as Bethesda would be crazy not to capitalize on the millions of potential customers who bought the game. When asked about future plans, Carofano explains, "Well honestly we're so focused on this DLC. Then I'm sure we'll be making some more DLC in the future, and then after that we'll see what happens."
There's a bunch of other neat tidbits in the interview as well (such as the ability to change your appearance at the Thieves Guild, included in Dawnguard), so check it out if your a fan of Elder Scrolls' latest continuation.