If Little Mac got in Smash.
I don't see why Doomguy shouldn't get in too.

Basically Punch Out is extremely unpopular in Japan.So much that back when Little Mac was revealed as a playable character,japanese Nintendo fans thought he was that boxer from Cpt. Rainbow.He was in side mission in a Wii game that came out only in Japan.He's THAT obscure there.
He made it mainly thanks to his popularity in America.

Doomguy on the other hand not only has his history with Nintendo,with an exclusive game on the N64 and with every game of the series(except Doom 3) ported on console but he's also an icon in videogames.

Plus the 3rd party characters represent certain genres. Sonic=Platformer;Snake=Stealth;Megaman=Shooting Platformer;Simon and Richter=Action Platformer;Pac-Man=Arcade;Bayonetta=Hack and Slash;Ryu=Fighting game and Cloud=RPG.

Doomguy would be an excellent rep for FPS.

I can feel it.

The need to rip and tear.

It’s coming.

Hell will come to Smash Bros.

And with it, the Slayer shall follow.