First some ground work:
PK (player killing/er): Refers to the act (or the person) of engaging in combat with someone who is not prepared or doesn't want to do so, and trying to kill him. Usually this is done by someone possesing a relatively higher level than his unsuspecting opponent. As a rule the attacked one will feel enraged or frustrated (depending on the loss that this means).
The original term is used widely in MMO games and is not restricted to the Diablo series. It will be used as described.
Duel(ing): Refers to a battle where all parts of it agreed previously to battle according to (or without) a set of rules. This is usually of competitive origin, though it can also happen from the fun of it.
Griefing: Refers to the act of anoying or bothering another random player in the game. Usually by entering a game and doing something to be a headache directly (staying AFK in D3 qualifies as that too).
PvP (Player vs Player): This definition can vary according to the situation. As a rule refers to a battle between 2 or more players. When applied to a realm/world/game refers to the fact that player's battle can (or must) happen. Depending on the game, PvP can be the norm or the exception.
This are the terms I will be using and I'd like those "hardcore PvP D2 players" to abide by these definitions since they are wider and more commonly used than PK=PvP. Don't start arguing because of this.
So many people seems to miss the old Hostile system in D2. While I didn't like it myself, since PK could appear just to be an !@#, I can understand the thrill that the system provides.
To addres the issue of PvP it's already known that Blizz will bring us arenas (hopefully sooner than later), but this is NOT equal to bringing back the hostility system.
My personal Idea to deal with this is actually very simple. Should you push the "Open game to public" button, they should present you with 2 options: PvP, Co-OP.
While both modes should allow one to progress through the missions together one should allow people to suddenly change their "attitude" towards you enabling combat at any given time.
This could make people who like flash PvP happy, at the same time keep people, who dont want to combat, safe (by choosing Co-OP). Also, this allows people who want to sneak on others, to do so, but keeping safe those that are not interested in being paranoic or just dont enjoy that.
To address griefing with MF Blizz should allow the game creator to have the possibility to kick another guy from his game, provided its just the two of them. An AFK timer could be of use too.
I understand why Blitz took out hostile mode (Pk/griefing//the way the game/story/maps progreses) But I think they should have taken a little more time to try and please more people.
Others suggestions are welcome