Multiple weapons with the ability to swap between them, perhaps with ammo to promote switching, and the ability to run and gun with most if not all of them
Would be a nice nod to/representation of his wide arsenal of weapons in the games. Attacks using the chainsaw could even spawn ammo. EDIT: Doomguy having a mega-man-esque run and gun gimmick is essential if he is gonna play like he does in his own games to any degree. He needs to be able to rush and fire at the same time.
I personally kind of imagine the weapon fire on neutral-B, with down-B being weapon swap. Side-B moves would be non-existent, to allow for run and gun, except for the SSG, which would have the meathook move as it's side-B. In this theoretical moveset, A-moves would be melee, armblade and chainsaw attacks. But he could be organized any number of ways if he actually happens, who knows.
The Meathook SSG attachment and quick dodge/dashes from Eternal
-The Meathook would make an excellent grappling rush move that pulls him to his enemies and adds to his recovery potential as well.
-His dodge could take on the properties of his dash move from Eternal, 2 charges, works in mid-air, only cools down when he's not airborne, would also provide recovery potential. The meathook and dashes could together supplement a potentially difficult and limited possible rocket jump up-B.
High Speed, Low Defense/Resistance, High Weight
Doomguy/marine/slayer could be a rather unconventional build if they draw from the original games while applying some additional logic. He lugs around armor, ammo and potentially 7 weapons or more, at least in the original games, so what if he's actually a heavy fighter? But unlike most other heavy's, he isn't super durable (because dodging is a priority in doom) and he IS super fast. He'd be hard to hit and hard to launch, but he'd have a hard time recovering once you do.
Just some of my ideas.