At least some maps, after level 100, should be open PvP (no drop/exp loss)
Just so people feel they are actually playing an MMORPG. Plus, the feature would attract a lot of new people to the game.
IMO, no PK mode or anything like that. If you manage to kill someone, they will simply return to the nearest location of respawn, and you wont win anything with that.
As I said, the idea is only to create more interaction between players / characters.