It seems like the ultimate "treasure chest" box would be a true treasure chest with the complete Dominion... all cards, all expansions, all promos... and add in a dozen or so kingdom cards that function in that "treasure chest" role, adding 1-2 cards based on each expansion mechanic. Yeah, it would weigh 20 pounds, and yeah, it would cost ~$300, but if the "treasure chest" as storage were well thought out and well made, it could solve the Dominion card storage problem and create the ultimate Christmas gift / collector edition for die hard fans.
Alternatively, sell the "treasure chest" empty. The empty version, at ~$100, would be a slick storage box designed to store all of the existing expansions, and would include the dozen or so new kingdom cards. This way, the "treasure chest" would appeal to serious fans and would also encourage fans who don't have all the sets to gradually fill out the treasure chest. And, of course, leave a few extra slots in the treasure chest designed to hold future promo cards or fan cards.
I think the "empty" version of the treasure chest would probably be met with huge response from fans as long as it was a quality product. Think sustainable hardwood, engraved logo, removable wooden trays for cards, etc.
It would also solve the request from fans for an official storage solution for all the cards, and the storage and new card dynamics would reinforce one another.