Hello,
I have an impression that any sort of restrictions outlined in the starting post will not change the situation in the long run. Besides the fact that, obviously, wealth is produced and accumulated, we also have some additional aspects like continuous emission of Fang with the almost invisible inflation, and other factors... I wonder if anyone modelled what a MMORPG-economy is like mathematically and what happens in the infinite time limit...
Why not introduce a (progressive) wealth tax system? And, as well, taxing of the businesses (say, traders in the outskirts)? The money collected by the system or (N)PCs would then be effectively given 'to the state', disappearing from the playerbase's hands. With careful plan-out, I suppose that the wealth level of even the richest PCs would become far closer to what they can obtain through a few dungeon runs for their level.
I understand that some would not want to pay taxes. This would give some RP-chances like hidden gold stashes or even 'offshores'... And I by no means suggest developers have time to do something like that!..
In a real life economy, the amount of money in circulation (whether it is hard coinage, or fiat paper money) is finite -- the Federal Reserve increases or decreases the amount of money in circulation in order to control inflation. When money is scarce, they print more, and when money is in surplus (and driving up prices as a result) they pull some out of circulation.
In a game economy, however, there is no equivalent to the Federal Reserve -- the game just keeps generating more and more loot, infinitely. One way to control for this is to have the loot drops start taking into account the total amount of gp held by the playerbase, and start generating less loot, unless more gold is paid back into the system by the playerbase.
The second prong of the system necessitates some mechanism by which players have to pay gold back into the system, and it should be relatively progressive so as not to penalize lowbies that don't have big bankrolls. When I was doing PnP long, long ago in a galaxy far, far away, we imposed a basic set of living expenses on players -- every month, a certain amount of gold was deducted to represent their typical expenditures in a character's lifestyle. We theorized that most adventurers live relatively high on the hog when they are flush with cash (it is the nature of swashbuckling adventure, after all), and we also theorized that the more powerful and renowned you were (i.e. higher level), the greater your attendant expenses would be -- this mirrors real life, when as you accumulate wealth, your tastes in cars, clothes, food, etc. tend to improve and get more expensive.
As a consequence, you could create a nominal deduction from everyone's gold reserve every game "month" (or however you want to set the recurring event), and base it on character level -- for example, you could say it costs 100 gp per level per month (just a hypothetical formula, by the way).
This would accomplish the goal of reducing the aggregate amount of gp in circulation, and provide an ongoing incentive to go out and adventure.
The only other alternative is to have more things that players will want to buy -- stuff like healing for example. Instead of healing being free at the temples, the priests could charge for the service. For example, the priest could cast cure light, cure moderate, cure serious, and cure critical as menu choices, and charge a player increasing prices for the stronger heals (sufficiently high, of course, to make the market in crafted potions more desirable). Equipment repairs at blacksmiths could also be an purchasing option. While I realize that this potentially detracts from player-to-player exchanges, having players exchange gold among themselves does not actually reduce the amount of gold in circulation and therefore does not reduce inflation. You need to take gold out of the system permanently, in order to balance the fact that the system constantly generates an infinite flow of replacement loot.