Years before Hearthstone there were the Physical Warcraft Card Packs. Yes a person may get lucky on sites like EBAY or something to get them, I just feel its harder to find someone IRL to play with. So my thought is why not turn them into Digital packs for Hearthstone. Possibly could even just keep the names of the expansions and names of the cards, just give any needed update to the graphics of the Cards.
As for any Mounts, Toys, Pets and those Arcehology cards in those Packs 1 Option would be allow people to sell them on the Auction House in WOW. Maybe even throw in some new items for people to obtain, for instance any event items like the Hearthstone Mount that may not be able to be obtained anymore and put it in there.