We've had lots of discussions regarding gold sinks and what to do with too much gold (this one recently)
and I have a (maybe) solution.
The idea is that all players could vote on something by spending gold. These votes would be mostly trivial and affect some in-game reward.
For example (now that we know re-skinning is easy and we can add new promos), each month there could be a vote for a new promo. Perhaps each month someone (could be devs, or just random decision) chooses 5 cards. All players can "buy" votes by donating gold in the marketplace, and whichever card has received the most donations will get a promo release. The 2nd and 3rd place cards will stay up for votes next month, and 2 new cards will replace the 4th and 5th place card.
This gold sink would be ideal because it would be completely optional, but players would also feel proud to know that they saved up a ton of gold and were able to influence the voting so that we now have a promo ice barrier or something Once a player has all upgrades, they have an easy way to spend any amount of gold that they want.