Other games have "Welcome back weekends" where old players can come back and play for free for a couple of days. In DDO we could have Free to Play Sunday every so often.
On these days all free players could sample any of the adventure packs without having to purchase them. This could encourage free players to find fun content and purchase it! We could also see the return of old players who gave up around levels 12-15 due to lack of free to play content.