Since the PvP phase is going to be the same as the PvE phase, but with player-on-player action enabled, I pondered how to make it work in a way that would represent a world full of heroes, villains, vigilantes and scoundrels.
And then the thought occurred to me that only villains should be able to attack anyone without provocation. Or, said another way, that attacking anyone without provocation is the act of a villain, by definition.
That thought lead to the next. What if in the PvP world, everyone was flagged neutral until they attacked someone. In other words, you don't start off as a member of any faction like in all other games with PvP. This would accurately simulate a real city with a bunch a chaos injectors, er... Player Characters.
Since the PvP phase would still have the same missions and streetsweeping that the PvE phase has, interacting with NPCs to either support or stop a crime would flag you as a hero or villain, regardless of your actual loyalties. Thus allowing other players the opportunity to attack you, thereby flagging them as the opposite, and so on and so on, until the domino chain of who attacks whom reaches its end. Anyone who attacks a hero or a neutral flagged player will be marked a villain until the end of combat regardless of how many villain flagged characters they also attack. So it is entirely possible for combat to devolve into a battle royale between a number of villain flagged characters each out for him or herself. Hero flagged players would have to be allies or else lose their hero status. (Hmmm.... this would make an interesting model of human system dynamic behavior!) After combat there would have to be a cooldown period until the flag goes away and you are flagged neutral again. The cooldown timer would have to be lengthy to allow people who play heroes to retaliate against the villain-flagged players who ganked them.
This way, PvP players would be able to choose what they want to be on a case by case basis. In other words, a player can be a villain one battle and a hero the next. Oh, and what happens inside a mission instance, stays inside a mission instance ;)
An elegantly simple emergent property of this is the true realization of playing a vigilante. So if there were some Rooks gang members just loitering, but not engaging in any actual criminal acts, would you be willing to flag yourself as a villain to complete your "subdue 10 Rooks" quest? That's what a vigilante or a villain would do. But a hero would scan the neighborhood and only subdue those Rooks who were actively participating in a crime: like B&E, purse-snatching or arson. A hero would often be handicapped by the very morals they claim to espouse, while a vigilante player would also suffer from the morally ambiguous life they lead.
The more I think about it the more I like it. But I'm too close to see any holes in it and would need other points of view to find them. (edit: how would friendly fire work for AoE abilities? hmmm)
Finally, I would expect that there would be some threshold of infamy that, once reached, would permanently mark you as a villain and you would be fair game for all heroes (and NPC) to attack on sight without being flagged as villains themselves. I would expect that there would be a bounty board for villains so infamous. Once someone reaches that threshold, their cooldown timer becomes infinite and they won't be marked neutral again until they start doing enough redeeming acts to bring it below the threshold, or until they are defeated and go through the criminal justice system. The converse would not be necessary for a heroic threshold. Since anyone can attack a hero at any time if they want to be flagged a villian, there would really be no need to flag someone as a hero in the same way as flagging a villain other than for vanity purposes, or maybe as a buffer against an inadvertent attack on a neutral. I suppose surpassing the hero threshold should also put you on the bounty board for villains.
This same flagging concept has been done similarly in other sandbox MMO what allow outlaw status. Archeage is the first that comes to mind.
Edit:(I posted this in the Let's Argue: PvP thread, but it was drowned by the deluge of posts in the ongoing argument of badges in PvP zones, so I'm reposting it here in a different thread)
Trap Spider - the little spider rides a big robot
Thunderbird Priest - conduit of the Thunderbird spirit
Lovebot - programmed to love man, gives tough love to mankind
Schwarzchild - singularity-manipulating time traveller is lost