I think that as a general rule, things people could conceivably do in real life should be possible just by leveling up skills. So, like, lifting a car. Believe it or not, people can and have done it in real life, so that might be allowable in real life. But taking a sword and slicing the car in two with one strike probably isn't possible in real life, so a player might need an upgrade for that. Also, most people couldn't keep fighting with a javelin going through their stomach. But maybe you could add a feat that lets characters ignore pain?