I would love to see open pvp that is done right for once. I usually prefer no pvp because most pvp in mmorpgs just plain stinks.
Open pvp adds to immersion, but in most games it encourages griefers, gankers, and all sorts of jerks. To take out most of this element, and make open pvp enjoyable, an mmorpg needs a balanced system of risk versus reward.
Possible Risks/Rewards:
Mystery Level
I think a person's level shouldn't be displayed to other players, and is only available by asking a person their level. A lot of PKers are simply cowards looking for amusement or an illusion of superiority, and get it by killing low leveled PCs. A lot of griefers would get deterred because they wouldn't be able to tell from sight alone who the low leveled people are. HJ excells in this, because armor is no longer an indicator of level either.
Law Enforcement
Both NPCs and PCs should take roles in the punishment of those who kill PCs in non war/duel settings. If a person gets killed by a PKer (not including consentual battles) then an NPC related to law enforcement should put out a wanted poster and reward on the PKer. People could then kill that PKer without fear of retribution from the law and get a reward on top of it.
Corpse Looting
With the first two ideals in place, this could actually be interesting. Grabbing a person's armor and weapon seems like it would be pointless in this game, but there is always money or other items. This will encourage more bandit/thug people, but it would also encourage others to enforce the laws as well.
Prestige
Lets face it. For some people out there, it'd be fun to be known as Gormax, Slayer of 1000 Men. I don't think this should be displayed on the character themselves, as I am personally against having people's names, titles, equip, etc being displayed by looking at a person or by /check them. It would be extremely cool to hear NPCs in a tavern talking about the most recent bandit king though, or the brave knight who took out five highway robbers all on his own. Good roleplay in that sort of thing.
I think this type of open PVP would really help to encourage roleplaying, but only if properly implemented. It would be cool to see people break off into non-ingame factions, like bandit and knight guilds. Well done open PVP also would provide gamers with a more interesting and realistic environment. Up to this point I can't think of any open pvp that has been realistic, simply because the risk didn't make sense or wasn't "risky" enough.
I realize what I ask for would probably be pretty difficult to design. If it can't be done, then I'd prefer consensual pvp only.