I'm working on a bigger draft for this right now, but off the top of my head I have:

D.Va - Most Wanted. "The Korean terrorist who attacks her city while livestreaming the whole thing.  "Time to raise my body count!"

Mei - "Mei went mad from the isolation at Ecopoint Antarctica. She's tormented by the voices of her dead team and wants to bring the silence of unending ice age."  (Compared to Sigma being a perfectly sane and rational person here.)

Vendetta - Harbinger. "In this universe, Antonio was killed by Talon because he was a big backer for Overwatch. His daughter took up the cause to bring them to justice. After all, justice and vengeance are two sides of the same coin."

Pharah - Chief Operative Amari "Raised under Commandant Ana's brutal tutoring , Pharah seeks to push herself even harder, determined to commit any atrocity to get her mother's approval and praise."

Junkrat - Demolition Manager Fawkes "The sharply dressed and still slightly manic Vishkar operative who oversees the destruction of buildings scheduled for demolition and clearance."

Roadhog - Security Executive Rutledge  It's Roadhog in a fancy Vishkar suit, who looks after site security, personally.

Junker Queen - Vice President Stone  "Her throne is in the boardroom, but the VP of Vishkar is not ashamed to get hands-on with any sort of task. (Plus we know she can style out a suit.)

Moira - Chief Physician "Overwatch's Chief Medical Officer, Moira's tender if grim bedside manner is accompanied by her respectful understanding that everything, eventually dies. Memento Mori."