The goal is to get the best of both Hardcore Mode Single Player and Anarchy servers. Basically when you first log in you are given a completely random spawn point (generate 2 random numbers between -5 million and +5 million, make those your spawn point x and z, maybe have a system where no two consecutive spawn points can be within a few thousand blocks of each other). Once you're there, it's standard vanilla survival mode Minecraft. If you die (or don't log in for a month) you start again with nothing at an entirely new spawn point. All your old stuff is now up for grabs to the first one who finds it. Yes, PvP is allowed. Yes, you're allowed to murder people to get their stuff. The massive distances and randomness of the spawn points should discourage repeated harassment and give killed players time to prepare in case someone does go looking to kill you again. I realize that this can lead to massive world save files, but I don't care. Maybe this can be done with console commands somehow, I'm not really sure.
Bonus points: If there is some sort of distance-based chat feature where only players within a certain number of blocks of you see what you put in the chat box, maybe with commands like /yell or /whisper to change that number of blocks.