The youtube channel Erickson Gaming ran an contest with their viewers and asked for them to share what characters in the Mario universe deserve their own stories, and I have to say that their community delivered with some great ideas! The top five that he shows off ends with a top fifteen list to give an honorable mentions list of those who's ideas were all runner ups.
This list is easily agreeable with and my personal favorites are Shy Guy and Professor E. Gadd from this list, but you can pick your own from the video: