We should be able to buy wildcards for gems. A simple idea that would work commercially for WotC and would give players more certainty in being able to build the deck they want, albeit for a cost.
I think a reasonable cost would be the 10x multiple of what players get for duplicates not covered by protection. That's not so cheap as to undermine buying packs, but within range for finishing a key deck.
So:
400 gems for a mythic wildcard
200 gems for a rare wildcard
Extrapolating from that, and adjusting for the Vault protection:
50 gems for an uncommon wildcard
25 gems for a common wildcard