Account improvements. Since the Server handles everything it's not important if clients are open source.
For example, "Direction to Boston for 24h" as unlockable account improvement. The clients asks the server for the direction to boston, if the account has unlocked the feature the server sends to the client in which direction an ingame arrow has to point. No other information, not how far it is and not how long it takes, only the direction.
You could sell additional character slots (are they limited? Don't know). Or the ability to directly trade between characters of the same account (that are not too far away from each other) in the character selection screen.