Dear @BlizzardCS
I've been thinking about a reward for those that finish an expansion in @PlayHearthstone (wild or standard).
Changing the entire collection of cards of the finish expansion to golden, (but like golden Kael'thas that can't be disenchated).

And the entire set of replaced cards got disenchanted but you only got ¼ of the total dust for the total expansion.

This doesn't affect the future expansions economy because, for example, even if I save all the dust for every complete expansion (no adventures) since "Goblins vs Gnomes"(™)

You can put a lock that only let the users to spend that dust for the hearthstone year they finish an expansion or when users receive the powder retroactively as soon this idea is applied in a future patch

If you apply this reward just before the year of the Phoenix ends, we can use the dust to craft the missing cards after opening the big bundle of cards, a portrait, a card back and a legendary card.

For the adventures you can do the same but without the dust, just changing the complete card collection in gold that can't be disenchanted.

About the Hall of Fame; you can only get the ¼ of dust when you get all the cards that are available are obtained.
(Until this tweet the Hall of Fame counts with 61 cards)

Ups, I almost forget it.
We can't receive dust for cards that have been a gift (C'thun or the 5 types of Galakrond for example) but having them will count for completing an expansion. (For those players that have those gifts)

The whole point of this thread about the golden cards that can't be disenchanted and the ¼ of the dust per finished expansion, is that users that like to collect all the cards get a little bit of recognition for the effort it represents.