Something I've seen is that older players (and those who just play an ungodly amount of time) tend to have more coin than they know what to do with. After you purchase all the forms out there, upgrade your vault to the max, and purchase a plot there's not a heck of a lot to do with your coin.
Something that could be done is implement an NPC that sells consumable items that you can get nowhere else. Having the items consumable will keep players looking for coin who would otherwise have no use for it. Making the items unique to the vendor will prevent this from devaluing the items that players produce. Obviously nothing should be overpowered and they should have level restrictions on use.
You could have the NPC sell some food items that have unique effects or existing foods that haven't seen the light of day in years because the formulas are obsolete (this would of course exclude seasonal foods as they already have their own vendors). Maybe some crush/slash/pierce converters that would be consumed on use could be sold where the duration and potency of the effect would scale with tier. Perhaps some deity-related items that give a relevant buff for a short amount of time.
Other than that, not really sure what to suggest as an example. So... thoughts?