This taken directly from FF8 triple triad, with the twist of using mini game components as currency. Basically do away with gold entirely. The new currency could be cards for example. Each type of enemy in the world has a card associated with it. When you slay an opponent he may rarely drop a card. These cards come in all sorts of grades ranging from critters to bosses, and the value of each card depends on its grade. You may wind up with a card from a boss, which would be worth a great deal. Since there is no currency to fill in the gaps, these cards will be highly sought after, and have high value since in number they would be more rare than a single gold coin. Everything you could have done with gold you can do with cards.

But now players can challenge, and win cards from each other . Since the currency is a piece of a minigame, that game will undoubtedly become popular.  Perhaps even allow to play vs NPCs for a limited number of games.

Players running around with boss cards could challenge others, and have an advantage but risk losing their most valuable card. Or they can trade, exchange, or spend it freely.

In FF8 there were 6 or 7 grades of normal cards, then there were mini boss cards, boss cards, guardian force cards, and character cards. In the same way faction heros could have ultra rare cards of them floating around. These would be extremely valuable since you could either exchange them for many lower cards to spend, or you could risk it as a powerful component to your card game strategy.

This could be applied to any number of minigame (and most mainstream MMOs are lacking in minigames)

The only issue is "what if you hate said minigame/s"?

Picture if you HAD to play tetramaster in FF8 in order to get gil for spending, and everytime you needed to make money you had to break from the usual aesthetic of the game and play cards. Now, this isn't necessarily a bad idea, I just say widen it to the concept of "only gambling makes money". That means any number of games vs the house (game) for set limits to gain, or with other players for high-roller wages. As long as you don't rely on *one* minigame that someone is sure to hate, as well as a need to make sure one game doesn't simply have better output and make it oversaturated is employed. How drops then relate to that scheme is up to you, but it's a good way to tie in assets to gamble with.

I would call it a "carnival economy".

You get dropped "prizes" from mobs, turn them in for chips/coins/tickets, then gamble or play games to win more so you can move up to better prizes.

How do you get gold in WoW?  You either trade for it loot it.  If the same were true for cards then how would the 3rd option of playing a minigame with your money mean that's suddenly the only way to acquire it?

Maybe I misunderstood you as suggesting the only form of making money be through loot trade-in's towards games of chance, and I still say it could work.

Edit; rereading it, you really gave a lot of indication that you were about changing over money drops for minigame tokens of any sort. Like, "the new currency could be cards".

I could see a lot of fun if you made the value of the cards to NPCs based on supply and demand.   basically the more the card is spent the less valueable it becomes,  the less often it is spent in prortion to other cards are spent the more valueable it becomes.

Then tie that value to individual "reigions",  which also indirectly means different types of monsters in that reigion.

Which means that cards from the same reigion you're in are less valuable in that rigion but a bit more valueable at the furthest reigions.   Atleast to NPCs.   And only by a slight amount if you make travel easy.

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You could even hide the value of cards a bit by doing something like this:

When starting a trade with an NPC you drag down the items you want.   Then  you offer cards until the NPC shows acceptance.   From from that point you can still swap cards in and out and the NPC will change it's views based on that.   If you make it a binary yes/no that means it becomes difficult to know the real value of the card at anyone time.

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In the end one person's fun is another's hell.