Well open PvP would surely not be possible everywhere, but I guess there could be some entire cities where it would be allowed, or there could be some areas of a town, of a forest ect.. where PvP would be allowed.
Let me explain my thought. If only some full cities had PvP enabled, it could easily not be among the ones players are starting in. A good explanation would be that there are and have always been some towns were there is only the rule of might. Put it together and you have your city with open PvP and some kind of organised crime. It could be chaotic from time to time, but the need to have one's back being looked after will get players to group in guilds or organisations and this will eventualy lead to some players made rules proper to each city with open PvP.
However, while this could be an option, I do believe it's still not verry realistic to limit open PvP areas to full cities only. In my opinion, it would be better to have some open PvP areas scattered a bit everywhere. Every towns usualy have their shady streets and places people don't usualy go unless they have business to do. It's the same with forests and other 'wild' areas. Of course full cities or places with open PvP could still be possible.
Now to avoid new players to get in such places in their starting towns.. I guess having walls around open PvP areas with guards at the openings to warn and prevent new players to get inside would be a solution. It would impossible for the new player to get through until he says the guard that he is getting there on his own will and knowing what's waiting for him ahead. This would happen the first times a new player tries to get in or be triggered during his first hours of play. Guards would give their warnings to anyone no matter what. The game guide could also give more in depth explanations the first time a player get near such a place. To also prevent them from being attacked as soon as they get in the said area, there could be a timer to prrotect them from open PvP (unless by 'challenge' way) or/and guards at the other side that would defend the player. The look of the open PvP areas would also be different from the usual city streets, to make it clear that they are dangerous places.
Outside of cities (or any other inhabitated place for that matter) the open PvP areas would still have a self explaining look about them, but nothing would prevent anyone to get in there. There could still be a timer preventing open PvP a server message stating that the area allows open PvP, but nothing more. After all, it will be the players faults if they leave a main road for exploration and such, and bandit raids are a part of exploration risks as much as ending face to face with an Ulbernaut.
Logging or loading maps in such areas would also enable a timer preventing the player to be attacked until it's sure he has finished loading and is fully aware of his surroundings.
Now this only considers open PvP, but to have the perfect settings, open passive NPC versus Player should also be defined. My opinion on the matter is clear. I think players should be able to attack any NPC anytime anywhere. Every now and then, people will bring the fact that if this ever happens some important NPCs could easily be killed over and over, which would disrupt any business players can have with them. What I would have toanswer is that NPCs don't necessarily have to be defenceless. There would be city guards ect.. I don't think Harnquist would mind smashing the head of someone bugging him with his hammer... Besides NPCs wouldn't be killed but only knocked out, and players can always heal them and help them. The best thing would be to have people attacking NPCs being flagged so that any player can attack them freely. This would be only fair.
There would be much less or even no guards in the open PvP areas (would also depend if the guards mentioned are normal city guards or just guards of a local faction of course), and there would be bandits and such. The NPCs there could be more or less important and more or less protected by their own guards ect.. Players would understand soon enough if they are hurting their own business and will protect some NPCs themselves (or try to get profit out of it or whatever else they can think of). One could say some players would attack NPCs only to get their powerful weapons or equipment. Well I don't think a skilled guard needs to be wielding an 'Ultimate Fire Longsword of the End of Times+13' in order to kill or stop some random player foolish enough to attack him. It's eay to get some reasons that would prevent such cases, and it's more than possible to make it not worth trying at all.
I think this could also be easily extended to stealing and any other 'unpleasant' skills. Someone who is holding some precious items and goes in an open PvP area can only blame himself if he gets robbed or get killed, since there would be no way to get in such places without being warned.
As always, RP should be the master rule no matter where you are, so open PvP shouldn't be a reason for having OOC behaviors or anything of the sort.
Now there should also be some ways of catching players and bring them to some sort of justice with penalities waiting for the outlaws according to the crimes they did. Such a penality system should be extremly well balanced so that players will think twice before doing something and so that it won't spoil the fun of RPing a 'shady' character. Allowing open PvP and passive NPC versus player everywhere would only be possible if every rules are extremly well balanced, and to balance them there will always be the need to test things on given areas were PvP would be allowed.
I'm not saying this is the best idea around or that those have never been thought out before. I'm only trying to give my thoughts on the matter, hoping that in the best case, the devs could get some ideas if any of them ever read that.
PS: seems like the post was a bit longer than intended