One thing that does hit me as a potential way to draw more people into buying packs is to have, say, a weekend a month where one of the packs is open for everyone, even those who don't have it. Perhaps limiting it to Casual/Solo and Normal difficulties only, and perhaps even a single run through it. If you already own the pack, you still have your normal access, and perhaps it would work to give something like a +1 to chest quality in those adventures.
Now, I have bought several adventure packs, but the reason I decided on them was largely because of the experience I had with them after finding a sealed box of the game in the bargain bin right after DDO went Free To Play. I'd gotten to mess around with a lot of different adventures I normally wouldn't have, and I kind of wish I'd had more time to explore and experience a few of the others that I didn't because I didn't have a good build and wasn't pushing for levels.
This could get some people to try the game more with the various packs that you might have a few open for the weekend (Ones for different level groupings to give everyone something to do, and perhaps limit it to a choice of one of the packs available). It would also have older players coming back to experience the content with friends who would be new to it.
And you would likely have the pack sell rather well once people try it. And if they don't, well, they still have the next months one to look forward to. Once you buy one pack, you tend to buy more of them, and other things.
Something similar that could work would be to temporarily open up the premium classes or races for a weekend, possibly limiting them to just Korthos and perhaps the Harbor, or level restricting them to a cap of 4 or 5 unless they buy the race/class, and the character may end up just sitting on someones screen after the weekend (Should drop to the lowest slot and not take one of the two free players get) but several people would probably buy them after trying them out.
And while the big named packs would draw in people to play (I know at least two people who considered DDO to try Deleras, and decided not to since that would end up costing more), the less used packs would get people to at least try them and up their sales.