evolutionary determination of xp rewards

Here's an idea I was playing with this week. It would work the best if there were many players connected to some central program. Since this would require some kind of MMO or, at the least, a successful phone game, I'm not going to be working with it anytime soon.

I was thinking about how difficult it is for designers to determine appropriate rewards in games. I'm going to talk here about the experience point concept, but it could be applied to any reward. I thought, what if the designer didn't decree what the rewards are? Is there some way procgen could be applied to maintain a balanced game?

The idea I came up with was as follows: Suppose you have an RPG. There is health in this game, and some method of healing. You could randomize xp rewards (within a certain threshold) and track the results by determining amount of health lost in combat. Any encounter where over 150% of health loss would increment the "too hard" counter, indicating the xp reward was too low. Any encounter where less than 25% of health lost would increment the "too easy" counter. Eventually you might be able to evolve xp gain that's appropriate to future challenges.

Of course, this would vary by player skill, but I think we could assume a normal distribution of skill. The real problem would be dealing with grinders, but perhaps there could be a preset or evolving "appropriate combat challenge" window in which this scoring method is applied.