I had been working on a server where PvP was set in "contested" areas... however, a question I posed went unanswered (as usual) so I stalled. I think it would be a great mixture of PvE-PvP if you went into, say, LGuk Assassin camp and PvP was enabled, but outside that specific area, it was PvE.
Kinda like the city arenas are.