I was thinking if you could make a plugin that delayed the damage taken from a mob contact. For instance, you could kill zombies and creepers with a huge ping and survive with full health. I doubt you can do this with the current hooks but still, just throwing it out there.
Ping has little effect on server performance, and the delayed damage from mobs will probably make the experience bad for your players.
Imagine mobs registering hits on players. After a delay player loses health. That would cause me to believe that there's server lag, initiating a series of rageeeeeeeee
Oh, actually I meant that the mob would have to touch the player for several seconds in order to start causing damage.