I've been thinking about this, and here's my "this just might work" answer.
Remove any and all DRM other than a simple "CD in tray" check or Steam-like authorization (one time at install, possibly periodic infrequent after).
Every game comes with a "key" of some sort, and register online.
That key grants the first 1 thru X DLC free for that user, even if it's total crap like the multiplayer "patch" for Bioshock2. If the registered user gets it for free, they don't care because it "came with the game" even if it came out later. Have an activation method for installing the DLC and authorizing it online.
Yes, this idea does hamper the second-hand sales market, but I'm honestly not as concerned about that. But what it does do is get people to buy the game and get a few DLC free (bringing them back to the game and getting another chance to get re-interested in it), while not being draconian and hampering the end-user.