The only heroes atm I think need immediate buffs are: Mei, Doomfist, Torb, and Bastion.
Sombra needs her ult charge nerfed, but would definitely need to be buffed or reworked in other areas.
Symmetra needs a rework 2.0 at this point. An actual rework, not just buffing her and adding two new abilities.
As for Junkrat, I don't know what they should do about him. Do you think grenade+mine should kill a 200 HP hero? If not, maybe they should increase his projectile speed so grenades and nerf mine damage significantly to make that ability simply to maneuver around and not to deal damage. RIP-tire is bullshit now, if you wiggle it around on PC it is impossible to hit, regardless of tier. The radius is fucking huge too.
Torb and Bastion do not need buffs. They can tip into OP category very easily. They are just situational at the moment.