Best solution imo would be:

1) PvP flag to toggle on/off (off by default, of course). If on, you can attack/be attacked by anyone with the flag on.

2) Request duel, already implemented in WD.

3) Joining fractions. Neutral players can't be attacked. And this could still have the PvP flag to boot. So even if your in a fraction you could still choose not to PvP.

The only "good" counter-argument for optional open world PvP I've heard is that people will harass you if you never do it. Ffs, everyone on the internet gets harassed, why even pay people that harass you any attention? The times I get harassed I simply ignore it. I could not care less about these kind of people and neither should you. Block/blacklist etc if it gets too annoying.

Also, no anarchy open-world PvP. That, imo, just sucks. Perhaps a dedicated zone for it, but not as a standard.