If an economy is to be established there must be a constant flow of money in and a constant flow of money out. Every basic government takes this approach. If we take the U.S for example, mints print money and introduce it into the system while taxs, and de-circulation pulls money out.
Without this flow of money an economy would be dead. Currently in game currency is only able to be earned by donating besides the initial amount you get.
If you try to establish an economy with this paradigm one of three things will happen.
1. One player will obtain all the money and no one will be able to earn more. This is due to the basic progression speed. The first one to advance and sell the most useful items people will be the one most players spend they're money with. Now because this player now holds all the money and is most
advanced, they won't be able to sell their items because he/she doesn't need any of them.
2. Players will forgo purchasing anything because they don't know if they'll get any more currency at all and rather hold onto it than spend it.
3. Money will exit the system as players trade and then quit until the economy withers unable to support active trading.
This happens if you put any limited resource into a habitat. How would you eat if you didn't know when you'd be able to get more food and you only have a limited supply vs knowing when you'll be able to get more food?
Now that we've established a baseline for need I'll make my suggestions.
Players are able to sell basic materials to the server for a low rate.
I.E: 64 diamonds for 10$. May not be a lot of money, but lets say a player has a item they need up for 200$, it wouldn't be too outrageous to simply go farm 20 stacks of diamonds to trade for the item you need. This facilitates both motivation to maintain player activity while supplementing the economy.
In-game time rewards. Two viable ways to do this.
Time ranks - 10 hours = X$
20 hours = X$
Passive currency gain -
10$ per 1 hour ect
If you were going to do this I'd recommend an afk kick timer for anyone bellow certain ranks, perhaps being another donation perk.
Voting money -
You vote, you get currency. Benefits the economy, benefits the server ect.
Mob kill count or boss kills -
Mobs killed BY HAND are rewards by a certain amount of currency.
Gives a unique way to generate currency while promoting diversity.
Perhaps expand certain sections - provide in game currency for people posting guides, giving helpful answers ect.
I said this was important as well. I would recommend either setting up a tax system or charging people to rent shops to facilitate the outflow of money
I could come up with more ideas if needed but personally I believe the only way for an economy to work is if there is an in and out flow of currency.