The game is going to have a few different types of currency, but here is the basic idea:
- Money - generic money for every-day transactions
- Gemstones - still generic currency but made for high-level transactions.
- Favors - special currency granted by the Highers. Favors are primarily used to purchase "luxury" items, and there are limits to how many can be earned in a day.
- "Raid Runes" - special currency awarded for completing high-level content, used to purchase high-level items.
Obviously names and specifics call for fine-tuneing, but that's the gist of it.
For the sake of simplicity, we're going to use Gold. specifically copper, silver, and gold coins at 100-to-1 exchanges rates(100 copper = 1 silver, 100 silver = 1 gold). Mined metals can additionally be smelted directly into coins. Coins are used for every-day transactions.
However, money has weight. This doesn't mean theres directly a limit to how much you can carry, but coins stack in your inventory just like any other item, have max stack-sizes, and have to be managed. This is where precious stones come into play. Every town will have a vendor who deals in pearls, rubies, diamonds, etc. These way considerably less and are thus easier to transport, but are also used in crafting.
Special merchants will only deal in gems and sell more expensive items, while smaller-time merchants will refuse to deal in gems(if you have a pearl worth 300gp, the guy selling healing potions at 2 silver a piece isn't going to make change for that).
Since these require exchange at an NPC, it helps us keep the money system relevant by removing any sort of hard-cap on currency.
Favor is a form of special currency handed out directly by the Highers. They can certainly be exchanged directly for money, but the more common usage will be for special "luxury" items. These will include cosmetics, deeds to houses, and blueprints for special vehicles/mounts. Additionally, items needed to "reincarnate" will cost favor. Very few if any pieces of equipment or weapons will be awarded from favor. This system is primarily for specialty things and quality-of-life enhancements.
Because RNG is a bitch, the idea is that high-level games and extreme-challenges offer a specialized form of currency instead of random item drops. The difficulty completed(and a certain amount of RNG) determines how many units of this currency you get for a run(could also do things like opts), and it can then be traded for items. Each dungeons or collections of related dungeons have their own form of currency and their own item list. There should also be some generic ones or some that specifically require runes from multiple dungeons.
Let's theorycraft for a second. Each raid has it's own "rune". Runes of a specific type can be traded for items from that dungeon; or runes can be traded together for items from a generic list. Then there is "rune crafting" where you combine runes of different types to get entirely new runes, which can further be traded for special items. It will be a complicated system to design, but would be a lot of fun.
The main goal is to remove or severely reduce RNG as a factor, but instead create so many different items that collecting them becomes part of the game in and of itself.