I agree with Kendaro - full out PK is a BAD thing. Look at RS - for a while it was \"PK anywhere\" and it turned into an everyone for himself, power-gaming atmosphere. Even with the Wilderness (the only PK zone in the game) it\'s still pretty bad. While limiting PvP to certain areas isn\'t quite within the bounds of reality, it would encourage those who are interested in roleplaying, rather than power-gaming. There should be someplace to fight other players though, simply for roleplaying purposes, i.e. guild wars and arena combat. As far as guild wars, you could have one guild declare war on another guild, and if the war is accepted, members from either guild can attack members of the other guild anywhere, anytime (but ONLY members of those guilds). Of course, then you\'d have to have some method of distinguishing who\'s in what guild, and making it so only those players can be attacked, and blah blah blah... well, maybe it\'s not such a good idea.
What do you mean by \"declaring hostility\"? I assume that\'s the same as \"challenging to a duel\". Duelling should be included also, with the option that you can ignore all challenges, if you don\'t want to be pestered all the time.