Allowing existing players to invest all existing cash into a unique physical item so they can carry it over into the new economy. The new economy should consist of a more complex monetary system that possibly involves fees or taxes for goods or services such as transportation. It should also include some form of bank account to safely store your money, as new mechanics may result in players losing what they hold on them upon death.
I am currently also looking into options of shifting the way how money is generated away from mobs, as mob farming can still easily be exploited. I generally oppose a permanent physical currency such as fake dollar bills or gold ingots, so new money may only be generated as taxes from successful trades/sales or as playtime-based interests involving your deposited money.