Mini-expansion 1: A small deck of extra dungeons/encounters/rumours and an accompanying book that incorporate expansion-specific elements to your RtL campaign. You could have a mini-expansion for each big-ish expansion (AoD, ToI, etc.) This would allow the expansions to play a more integral part in the campaign without "forcing" people to have game components they could not use. These mini-expansions, since they just consist of cards and a book, could be very reasonably priced.
Mini-expansion 2: Blank cards for dungeon floors, Rumours, Encounters, Treasures, etc. to allow people to create their own and seamlessly add them to their campaign.
Thoughts?