Doomguy's most gory acts are reserved entirely for his execution "Glory Kills" mechanic which is obviously not going to be included in a fighting game (that isn't MK at least). Otherwise he is a punch-shoot man that uniquely relies more on physical up-close violence compared to the rest of them. Considering we already got a "sanitized" version of him via the Mii Outfit, I don't think giving him more accurate weapons would be crossing any lines we haven't already crossed.
And for comparison sake, Bayo "I should have been a pole dancer" Netta's games have not just a similar style of executions with not just blood, gore, and violence but a double dose of sexual theming and religious blasphemy:
and obviously that's not in her Smash appearance.
So while Doomguy might not be very likely, I don't think he pushes the bar in terms of content warnings.