The free DLC thing has one flaw- There are still people in today's society with no internet access. So, they would get no benefit to buying a new copy when a used one would work just as well. There is no reliable way to reward this group of people for buying new without putting exclusive content on the disc (and depriving used buyers of content that's already on the disc is a horrible idea). Other than that flaw, it's a great idea.
So, how about this. The developer releases two versions of the game. One version like the normal versions you see today, at full price with $10 of extra "not-already-on-the-disc" DLC, and one version that's the price of a typical used game, without the free DLC. You can still trade in either version, although once the DLC code in the "Free DLC" version is used, it becomes the exact same as the used version.
Basically, I'm suggesting that developers try to compete in the used games market without just complaining about it. It wouldn't be too hard- Would you rather pay $50 for a used game, or $50 for the same game, but new?
That would probably hurt game stores and such, but it would shut the giant companies up for a time, and would certainly be better for the consumer than locking out vital parts of already-on-disc game to used buyers.