I agree that it sucks losing LP because you had an AFK on your team. I heard that Riot is currently working on a system that will cancel/redo a match if a player fails to connect from the very beginning. However, there's not much, if anything at all, that can be done to compensate for having a teammate leave in the middle of a match without creating other unfair scenarios.
Also, losing LP based on how bad you play and gaining LP based on how good you play isn't as good of an idea as you may think. Gaining/losing LP based on your individual performance would pretty much completely change how ranked is played. In short, it would basically create situations where players would try to maximize/minimize their LP gains/losses instead of actually trying to win the game.