Nah, the system is perfect the way it is. If you go for an assassination, you should know the situation that you're putting yourself into. Invulnerability or higher health would ruin the risk-reward factor and makes the showstopper medal impossible to get. Although, the only thing I would change is that if a teammate kills the person that you are assassinating after you've delivered the killing blow, you will still receive the assassination, and your teammate would get an assist or maybe something new, like an assassin assist medal. Can't kill a dead person in a sense. But if your teammate kills them before the blow, then you would receive an assist and your teammate would get the kill, plus the yoink medal.