About Question: No I don't think he really deserves to be in this one. He is a fantastic character that deserves more love and recognition, but this maybe isn't the right place to do it. I think he deserves a TV show, or film, or even a solo game (LA noire with punching?), but a fighting game with all these heavy hitters I don't think so.
About Slenderman: He hasn't been in Supernatural because he is technically a copyrighted character.