Some mobs already have hate resets. They are not necessarily working properly or all the time but things like "wasp" and "mole" mobs that teleport/burrow are hate reset mobs that go after the person that hits them worst after they disappear. The problem is they can be hit by late attacks so based on who actually hits them the most after they disappear is who they go after. If they work on that system more and add a few more mobs that have hate reset abilities I think the tank will have plenty to worry about.