No! No! A thousand times no.

Open world PvP in an RP game is just a bad idea. It redirects dev effort from places where it would be better spent, and creates an environment where thrillseekers and gankers can thrive... and once you open that door it's going to be really hard to close. The problem comes down to if you take over competitive people, put them in an anonymous environment, then let them use that inherent over competitiveness to crush others in totally unfair fights... they will do it by any means available. And you don't want that.

To use a sports metaphor, there's a reason you can't just have 10 people on your basketball team. It would be stupidly unfair! What, will the others guys show up with 15 for the next game? This is "open world PvP", it's not a fair fight. People will seek whatever advantage they can, be it lowbie slaughtering or 2 on 1 fights just because that's the easiest way to "win." And winning is what's important to overly competitive types.