
Seisho |

Doomfist - most likely Soldier with Mercenary bg
Genji - Operative with icon bg (he was kind of a vip)
McCree - soldier bounty hunter
Pharah - soldier mercenary
Reaper - soldier/operative mercenary
Soldier 76 - soldier mercenary
Sombra - technomancer, bg is kind of hard
Tracer - operative with ace pilot
Bastion - android soldier mercenary
Hanzo - soldier/operative bounty hunter
Junkrat - soldier mercenary
Mei - possibly envoy or mystic, scholar
Torbjörn - mechanic mercenary
Widowmaker - operative icon
D.Va - soldier icon
Orisa - android soldier icon
Reinhardt - soldier mercenary
Roadhog - soldier mercenary
Winston - mechanic or technomancer free bg
Zarya - soldier bounty hunter
Ana - soldier&operative with custom equip, mercenary bg
Lucio - envoy icon
Mercy - mystic, scholar
Symmetra - mechanic or technomancer, scholar
Zenyatta - android solarian priest
btw I totally plan to have 'Watchpoint Absalom' in my campaign, my party will be new overwatch members

The Sideromancer |
As somebody else who doesn't have the book, drawing from info around the boards:
Doomfist: I remember a blitz specialization for Soldier, mercenary theme
Genji: Outlaw Operative (acrobatics/athletics to Trick attack?)
McCree: Bounty Hunter Soldier (unsure of style)
Phara: Mercenary soldier (bombard)
Reaper: Outlaw Operative (Ghost)
76: Soldier or Exocortex Mechanic, and unless you can retrain your theme, he's still the Icon Jack Morrison
Sombra: Outlaw Operative (hacker) or Technomancer
Tracer: Ace Pilot theme, I think Solarion had some nice speed powers.
Bastion: Xenoseeker Soldier/mystic on a custom race
Hanzo: Phrenic Adept Bounty Hunter Operative/Soldier
Junkrat: Mercenary or Outlaw Soldier
Mei: Scholar (I'd have to check specifics)
Torbjorn: (unsure of theme) Mechanic
Widowmaker: (unsure of theme) Soldier (sniper)/operative
D.Va: Icon Mechainic if they get power armour options, soldier otherwise
Orisa: (unsure of theme) Envoy
Reinhardt: Bounty Hunter or Mercenary Soldier
Roadhog: Mercenary Soldier (?)
Winston: Scholar Technomancer or mechanic
Zarya: Icon Solarion or Soldier
Ana: (unsure of theme) Operative (not a massive damage dealer with sniper weapons, but good at ranged debuffs)
Lucio: Icon Envoy
Mercy: Scholar Mystic
Symmetra: Scholar Technomancer
Zenyatta: Priest Mystic