Here's a great PvP system:
First, this requires a bank in the game. Here's how it works:
Kill a player, take his stuff. The value of the stuff creates a bounty on your head, of the same value. If someone collects the bounty, they don't just get your stuff, they also get the value of the bounty on you, from your bank account. If your bank account won't cover collection, then all loot (items, coin, valuables) you get from that point, disappears, as a repayment of the money owed, until the bounty is repaid.
Charity to other players could also be tracked, and be subtracted from future bounties, as a sort of paid "karma'. ahead of time.
Here's why it's awesome: Big players can level up, get their hp, armor, weapons, etc, and become super awesome. However, if they become big bullies, they'll end up with such a bounty on their heads, that the newbs will be hunting them em mass.
(If anyone even needs to ask... No, you would not get a bounty on your head, from collecting a bounty.)