Different color stars
I love organizing by color. I also love to use the stars to mark important emails or emails that need my attention more. What about offering stars of different colors for those of us who like to color code? It can be an option in settings so those who don't want to do it don't have to, while others can opt in.
I thought of this because I use my email as a To Do List a lot, and I wanted a way to "group" things together, if that makes sense.