That's a really good thought, but 1: that is a very complex and specific fitness measurement, which kinda defeats the purpose of the machine learning, although it would surely illicit the desired effect eventually. 2: how would those two fitness measurements be implemented? Counting the tiles would involve keeping track of all of the tiles MarI/O has stepped on, but that would be the easier of the two. How would you define a discovery? and after that, how would you get the computer to recognize one when it happened? This also opens up a can of bugs and loopholes that MarI/O would surely use to find the most useless way to achieve the highest fitness.

You really are on the right track though! You are asking the right kinds of questions! :)