Just a few ideas.
Something that I think would be cool to do is make it so that when a player has .5 HP their movement speeds decreases, making them unable to run. This would promote Pking and make it less irritating. Note - this would not effect spar.
If a player is killed by use of certain NPCs (Spikes in Sardons), then the player that did the last hit to them will be given the kill. Very annoying when pking sardons and players purposely run into the spikes to prevent you from getting a kill. If you ask me, the spikes should be removed completely, but this isn't the thread for that.
Thoughts?
Post your own Pk-related ideas!