Let me make some suggestions and see if they would be feasable in DIII:
1.) Simply have an eable/disable PvP upon map creation. If people want the risk, they can have it. IF people DON'T want the risk, they won't. IMHO, why force someone to PvP that really doesn't? The only ones that complain about it most, that I have seen, have been the griefers. This function limits their "prey" to just the people who want that type of PvP excitement and most of the time they are often more challenge than the griefer wants.
2.) ADMIN FUNCTIONS: Whoever creates the game can make changes on the fly. If somebody griefs, then they can be kicked and/or banned. I know it could be abused. Say if somebody was kicked because the admin didn't like the fact he got a good drop and the ADMIN wanted to claim it, make it where either the player can recover that loot later or just make it disappear so that no one else can get it.
I've heard the suggestion of the PvP enable switch mentioned MANY times, but never about admin functions. IDK, some enjoy the open games. It adds extra challenge, but I just don't see why people would want to make everybody else play that way that DON'T enjoy having the risk of being PK'ed when the least expect it. PW locks only work for just people who want to play with friends they know, clan meetings, etc.. It's like locking the doors of your house to keep theives out. Makes one paranoid in the end.