Open world PVP zone outside of a Instanced area like Bozja or etc would only end up doing more damage than good. You'd have people in groups going around killing just about anyone who might be doing gathering, treasure maps, side questing, main story questing etc. It's the grim reality, give people the tools they will take advantage/exploit the means. Having a pvp zone be a instanced area would be better suited given players are given the warning "This is a pvp Zone" before adventuring in it.