There are some things in wrestling, based upon a person's character and physique, that should never be implemented. Only because the same concept has been tried a dozen times before, and it never works.

Rey Mysterio would absolutely never work as a Heel. For starters, everyone loves to cheer the little guy in feuds. His wrestling style also is designed so that fans cheer him.

Other concepts not worth trying, because they typically fail:

1) Large Men very rarely get over and are successful at being Faces.

Like Giant Gonzalez, Great Khali, and even Big Show ... who was also not very successful as a Face.

2) Wrestlers who don't speak English also don't make very good Faces, either.

See Kamala, Great Khali (again), Hakushi, and Umaga.