Going all the way back to Season 1, I felt that Dean’s girlfriend Cassie was a little overlooked in 1.13 “Route 666” but the character, in my eyes, with the most wasted potential would definitely be Missouri Mosely, the psychic who appeared in 1.09 “Home”. Similar to how Cassie was replaced by Lisa as Dean’s most favorited love interest, Missouri was replaced by Pamela as the boy’s psychic friend.
Pamela was a younger, sexier character who the boys could flirt with, but I felt that she was very plain and stubborn. Missouri didn’t take no sh*t and had a blunt, straight feel to her whilst simultaneously being a comforting mother figure for them, especially to Sam who was also going through some very scary, psychic problems of his own at the time and needed some guidance. I felt that Pamela was no where near as caring and loving to the boys as Missouri was and, instead of bringing in a new character, the writers could have stuck with her. I think that she was very similar to Bobby in terms of attitude and emotional impact on Sam and Dean, and she could have easily been a big part of the story like Bobby was.