Actually, one question, because I played easy mode again and it still feels perfect to me-- by "one type of enemy" do you mean only one color, or only one size?
Hm, would switching between two colors really be much easier than switching between three? You only have to pick one tower color per wave, so it seems like it would be relatively the same amount of challenge, just with one fewer menu option.
But if a lot of people asked for the color red not to be in easy mode, for example, I would definitely do it. I just need more data and opinions!