I've always wanted everything's damage to scale on velocity. Not just the velocity of the damage source, but also the velocity of the damage target.
This way, if you're moving away from a melee attack, you would take less damage, where if you move towards it, it's going to hurt you more. The same could apply for player to player collision. If you jump off a cliff and land on someone, you should both die. Of course, ignore damage all together if the players aren't moving fast. Probably a max of like, 3 damage, if they both walk into each other, kind of like bumping heads by accident.