Even if I hate him ingame for me it’s gotta be Genji.
But that might also be because I love stealth games so I am really biased towards stealthy characters (ofcourse he isnt really stealthy ingame but he literally is a cyber-ninja).
A singleplayer game would also allow for a in depth talent tree since now they only gotta worry about one character + PVE balance can be a bit OP.
As far as I know singleplayer games aren’t really blizzards thing though which is kind of a shame. I think PVE in general would have been much better as its own game so you dont have to worry about balancing issues.
I mean reworking a hero when that hero also has a talent tree sounds like a nightmare to me.
Rein in his glory days.
Shooting Star gave us an idea of what a d.va game could look like.
Kiriko could be fun with fighting off the Hashinoto gang.
Ashe and Cass have Western themes and could be in the same game. Play as either Ashe or Cass, and by the end, you have finished off the other.
Genji would be a fun rogue-like
Ana or Widow would be cool assassin/sniper games
Moira would be funny for some type of create monsters to fight your enemies game.
A lot of the above use lore related aspects but it doesn’t seem like OW did much for these. You could go and say it is like a story mode that focuses on each hero.