doomfist could be viable if
1 - doomfists rocket punch can aim up and down instead of just forwards. you could attempt to take out pharahs and possible jump up and rocket punch down in an attempt to hit reinhardt. it would force reinhardt to shield up and leave him vulnerable.
2 - his gun has more ammo and shoots at a consistent rate.
3- no cooldown on uppercut since everyone else gets to shoot as much as they want but all he has is melee.
Now im not saying all of them at once. but these are just things that could be considered
4 (an edit) - Vastly increase his movement speed