Let’s see if I got this right…

Orisa – Mix of Metal Gear REX derivatives and predecessors (RAY for her right arm and forelegs, Shagohod's missile pods on her back, Peace Walker for her hind legs. Unsure about left arm.)
Mercy – The Boss
Zenyatta – Psycho Mantis Screaming Mantis (Forgot about her multiple arms)
D.Va – Liquid Snake
D.Va’s Mech – Metal Gear REX
Pharah – Raging Raven
Bastion – D-Walker
Ganymede – Emma’s Parrot(?)
Roadhog – Vulcan Raven
Sombra – Mistral
Junkrat – The Fear
Zarya – Meryl
Torbjorn – Big Boss
Winston – Otacon (hopefully)
Reaper – Vamp
Symmetra – Fortune
Lucio – Fatman
Genji – Cyborg Ninja/Gray Fox
Reinhardt – Solidus (I also considered Khamsin, but the P90-styled hammer gave it away)
Hanzo – Sam
Ana – The End
Tracer – EVA
McCree – Revolver Ocelot
Mei – Raiden
Widowmaker – Quiet
76 – Solid Snake

What? It was obvious.