Bloomberg News suggests:

Four changes turn the classic game into its deregulated — and more educational — variant:

1. Let every player borrow and lend from any other player. They are free to negotiate all terms, such as the interest rate, the repayment schedule, collateral and down payment requirements (if any), and what happens in the event of default. This allows players to borrow before buying any property.

2. Players who lend cash in exchange for pledges to be repaid can use those promises to secure loans from other players. They can also sell them.

3. Let players sell shares of the rental income generated by specific properties. The stakes can be traded and used as collateral for loans. Players can set their own rules about dilution and how to finance additional investments on their properties.

4. Let players write derivatives contracts with other players, such as insurance policies that pay out when a player lands on specific properties and options contracts that give a player the right to buy un-owned properties at preset prices from another player if he lands on it first.  ReWriting Monopoly