Introduction
This thread was originally intended to only have one suggestion but as I wrote, more and more suggestions came into my head, therefore this thread is titled as my mass suggestion thread.
Referral Program
A referral program that allowed players to recruit other players for a mutual reward would be an easy way to get more people to join the server and it's free advertising. To refer someone just type ::refer to get your code and when someone joins they can type it in to receive the reward. Players should only be able to refer a max amount of players to avoid abuse of the program, and obviously same IP's should be blocked. The reward should stray away from cash to avoid adding more money into the game. A new item such as a referral book (could give one option or a few options such as; bonus xp, pk points, vote points, donor tokens, etc...) would be a perfect reward, otherwise a vote book or 5m cash could be sufficient.
::Yell
Allow normal players to use the ::yell command with a time limit but charge a small sum of cash (around 100k-1mil) therefore if a normal player has something to important to yell they can still yell but it's also helping the economy by taking out some cash.
Trading Post
When buying items you should have an option to buy the max you can afford. When pressing the "buy all" option if you don't have enough money it won't buy any so you are forced to calculate how much you can afford.