Note that this will not keep the game 100% fair, but I think that's ok for Monopoly, since as we all know in real life some people have certain advantages others don't. These are simply suggestions, and I encourage you to select from these and modify them yourself.
The Banker is their own player, and is the "god" of the game, kind of like a D&D Dungeon Master. They can't win or lose, and they have only 2 objectives. Those objectives in order of importance:
- Keep the game fun and exciting.
- Keep up the Bank's cash flow, mainly through bank loans or mortgages to players. This doesn't actually do anything for the Banker, since his or her funds are inexhaustible, it just enhances the role-playing.
- Start with one these options:
- Action off starting order.
- Roll for starting order. Players can then sell their place in line if they wish.
- No property purchases until once around the board.
- The 1s or even the 5s are a waste of time, just round up to the nearest denomination.
- This is an actual rule in the game, but it's often neglected: auctions for properties that are landed on but not bought.
- Free Parking is a lot of fun. Use it. All fines go to the center of the board.
- On occasions where a token lands on the same space as another token, in addition to the normal actions that would happen on that space the two players each roll a die to simulate a "battle". The low roller pays $50 to the winner and puts another $50 in the Free Parking pot.
- Make sure to enforce the doubles = roll again rule, and especially the going to jail on 3rd doubles rule. You could even lower this to 2nd doubles sending players to jail.
- No collecting rent while in jail, making any deals, or buying one's way out of jail. If you don't have a Get Out of Jail Free card or roll doubles on your third turn in jail you pay a 10% fine of your assets and move the number of spaces you rolled.
- Double cash for landing on go.
- This is an exotic rule but one with a potential for a lot of fun: eminent domain. If player A wants player B's property and player B is unwilling to sell, allow player A to force a sale by offering cash to all the other players in the game. If the other players agree to the takeover player B loses the property and is compensated its face value, while player A distributes and equal amount of cash to the others.
- Replace some of the junkier cards (pay $15, collect $10) with more interesting ones: a game vote on who to send to jail, jump to any space, etc.
Some of these rules were my favorite picks from Jasper Fforde's lovely Monopoly X-treme.