As well as a Pking server, have some Non PVP servers. Or have some sort of punishment for Pkers that kill without reason because I hate MMOs where a lvl 99 (so to speak) is standing outside town killing everyone who leaves.
One of the better MMORPG systems for PKing I've seen is where Pkers who attack others become marked and if they get killed, the player who killed them would have a temporary hero status which gives them better drops/gold rate etc.
Of course in a PVP server that wouldn't apply since all goes

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I've never played Ultima Online, so I don't know if they have this, but have a buddy/friend system where you can IM/chat with the person. I know this sounds like a guild system, but I'm thinking more of just a chat type program in game in case a friend isn't in your guild or something.