I kinda wish the Green Knight had earth powers. I'm not a fan of poison effects, but oh well. I picked him anyway when I first played this because green is my favorite color.
That being said, it would be cool to have a knight with earth powers. As for the topic, unfortunately, I don't have much to offer for a yellow knight, but I do think there should be some knight of yellow. Maybe not have elemental powers. Since there is apparently a purple knight of sorts the game now has knights related to all secondary colors. If we get a yellow knight, all primary colors will be represented!