As for Doomguy, I've been supporting him since before Doom came to Switch. Even if you take out the visceral elements of his Glory Kills and don't let him use shotguns (or alter them to make them less realistic), he still seems like he'd be a lot of fun. I feel like Cyberdemon would be a better fit for an assist trophy than Olivia Pierce, though.
I believe Cyberdemon would fit perfectly as a boss character if there's Boss Rush mode or I dunno, an actual story mode(like Subspace Emissary) for Smash Switch.
Speaking of a Boss Rush mode since we never got one for Smash Wii U/3DS, I was thinking about some ideal characters for a new Boss Rush mode that I would love to see(Yes, a few will return):
Peter Piranha from Super Mario Bros
Great Reaper from Kid Icarus: Uprising
Porky from Earthbound
A Guardian from Zelda: BOTW
Ridley from Metroid(I know people will say "Oh but Ridley deserves to be a fighter", but we don't know yet.)
Kraid from Metroid
Mecha-Bowser from Super Mario Sunshine
Andross from Star Fox
Metal Head from Xenoblade Chronicles
Chaos Kin from Kid Icarus: Uprising
Cyberdemon from Doom (OBVIOUSLY!)
Dark Matter from the Kirby series
Hades from Kid Icarus: Uprising
Master Hand and Crazy Hand(No duh.)
A new baddie for the next Smash game. Like Master Core from Smash Wii U/3DS
I couldn't think of a Pokemon that would fit as a boss.
Anyways, that's those characters I like to keep an eye on for the new Boss Rush if it comes back.