This post discusses implementing world PVP and using a karma system and director AI approach within the player encounter/server logic to create an exciting and frustration free player experience. I am a bit verbose, so TLDR at the bottom 
First, Iād like to give props to the Diablo 4 team for reworking itemization and stat/skill tree concepts. I was pretty put off by what they had initially announced in regards to those categories, and frankly wasnāt very excited for this game. The most recent update clearly shows they took community feedback and put in the work to develop much more intuitive, creative, and fun sounding mechanics that now have me extremely excited to play.
That being said, now that the core of the game is coming together better, it has my mind thinking about other aspects of the game, and at the moment, PvP. PvP is something I feel most online RPGs these days just donāt do very well. I havenāt been hooked by many PvP systems in an RPG. What I really loved about PvP in D2 was how fast paced it was. Youād step out into the Blood Moor and it was like a Dragon Ball Z battle, with people teleporting at high speeds, dodging hammers, bone spears, arrows, just trying to land a killing blow. It was such a rush.
WoWās PvP is probably my number 2, and primarily PvP that took place in the world, not the battlegrounds. Guilds raiding entire areas, wreaking havoc, killing NPCs and players alike. ArcheAgeās PvP would probably have been better than WoW, had rampant hacking not been an issue. The gaming industry has long accepted a flawed premise in their approach to PvP, and that is, that a game where someone can kill you at anytime is not appealing to the mass market, and so in order to make the game accessible to all, they just remove world PvP in most cases, or make it in some way optional in others.
We can probably all agree that we donāt like being griefed. Nobody wants to be getting camped while they are trying to make some progress in a game, especially if you donāt have all the time in the world, and the hours you get to spend gaming are precious. I never personally have had many occurrences of being griefed in D2, where anyone could hostile you at any time, but alas times have possibly changed, and communities now are more toxic than they used to be. Also, Diablo 4ās player encounter system will be much different than D2, so D2ās PvP system wouldnāt really be an exact fit.
Some of us can agree though, that we like world PvP. It stimulates emotion and creates a sense of excitement when you come across a mischievous looking character, unsure if you should prepare for a fight, launch a pre-emptive attack, or if you will breathe a sigh of relief as they choose to spare your life and just be friendly. It increases immersion, because just as there are good and evil people in the real world, the same exists in the game. It makes you even more grateful for the good players, and the evil players at least serve to break up the monotony. Evil players also give good players a sense of purpose. It is satisfying to save weaker players from a strong predator, teaching them a lesson theyāll hopefully remember. It even motivates players to get stronger so they can be the ones to stand up to bullies, and allows weaker players to look up to righteous stronger players as an example of what they might become if they keep playing.
There is something unique about Diablo 4 though that creates a new opportunity for PvP unlike most other online RPGs, and that is how it handles player encounters. So far, not a ton of information has been released about it, but the general idea that has been communicated is that you will randomly encounter players as you explore the world. When you are exploring an area, you will not see every other player in that area, but rather a selection of random players, almost as if the area is a public instance that you are randomly assigned to. This is significant because this means the game can control which players you come into contact with.
In video games, there is a concept of AI referred to as a Director AI. The job of the director AI is to create an enjoyable experience for the player. This means balancing the difficulty of the game in a way that keeps it fun and exciting. For example, if you just won a really hard battle against all odds, a director AI would recognize this and choose not to spawn another really hard enemy as you return to town to heal up. The idea is that is a game just constantly screwed you over, it would not be fun. If it constantly handed everything over to you easily, it would also not be fun. If the game was non-stop maximum intensity conflicts, and never gave you a chance to breathe, youād get burned out. You get the idea.
Alright, so now to tie all these ideas together. I want to see world PvP in Diablo 4. The game itself could be designed to control player encounters in a way to prevent griefing and to prevent a frustrating experience. This could be done by introducing some sort of player karma metric. Players who frequently kill other players could be marked in some way so that other players can see they are evil. Players who kill evil players receive good karma and could even be rewarded in some aesthetic way for achieving certain karma levels. The director AI for player encounters would then be able to control who encounters evil players. Evil players could have their encounters limited to only encountering similar strength characters to prevent griefing of lower levels. Evil players might be less likely to encounter neutral characters who abstain from PvP (less likely, but should still be possible, else it would be stagnant and boring). A player who is killed by an evil player could be prevented from encountering another evil player for a time period. And then of course, there should be a way to consensually duel people, which would have no effect on their karma rating. Displaying who is evil, or if someone turns hostile against you, also gives players a chance to flee or brace themselves, which I think adds some fairness to the whole experience.
There is obviously a lot more that can be fleshed out of these foundational ideas, but I really think that Diablo 4 is in a unique position to create one of the most satisfying PvP experiences to date.
TLDR:
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World PvP creates an exciting, emotional, and immersive experience.
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The primary problem with world PvP for most is constant griefing and camping.
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Diablo 4 introduces a unique player encounter system where all players in the same area are not in the same instance, but will randomly be assigned an instance and encounter other players also assigned to that instance.
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Diablo 4 could introduce a karma system to track evil PKers and mark them. Also provide good karma to those who kill evil players, and award karma levels with some aesthetic options.
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Diablo 4 could then use a director AI concept within their random player encounter system to control what types of players you encounter. Low level players wont randomly encounter stronger evil PKers. Players recently PKād by an evil player wont encounter another evil PKer for an amount of time. Evil PKers are less likely to encounter players who typically abstain from PvP.
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The goal of the director AI would be to control your random player encounters in a way that creates an enjoyable and exciting experience, while not allowing the experience to become unreasonably frustrating.
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Include a consensual dueling system as well which exists outside of the karma system.