I am not as hard on Steve as others, but Master Chief's popularity always kinda bugged me when I always figured Doomguy would be a more fitting FPS rep with his even more important role in FPS history and surprisingly good Nintendo history.
I still can't believe people think that a generic military marine (from a series of games that has 0 history with Nintendo) is more fitting and has more chances than a cartoon character from a series of 3d platforming(who was 2nd Party and, until this day, is still kind of considered a Nintendo character, despite not being owned by Nintendo anymore)!
I do agree that Doomguy does fit in a weird way (much like Solid Snake)!
He is iconic and his most popular games are classic and vintage titles that even revolutionized the FPS genre!
Despite being violent and gory, his games (specially the first ones) are still over the top in kind of a cartoony way!
Plus the demons from his games have a really colourful and creative design that kind of translates well with Nintendo's style!
If he does get in, I just hope they go with his classic armor suit, the new one is way too generic and uninspiring!
Still, the only M-rated character (or hell, the only videogame character) that I trully want in Smash right now, is Conker!