If anyone, it will probably be one of the Original Overwatch members.
If i had to take a guess, this would be from highest to lowest chance of getting killed;
- Soldier
- Reaper
- Reinhardt
- Winston
- Tracer
- Ana
- Torbjorn
- Liao
The most iconic Overwatch members, basically.
As for why i think one (Or multiple) of those would get killed off?
Because that would inspire the rest of the Overwatch team to defeat the big evil to avenge their fallen comrade, and these 8 would have the biggest emotional impact.
Another possibility is that the older generation passes on the torch to the younger generation, by sacrificing themselves.
In Reaper’s case, i’m guessing that he turns out to be one of the good guys at the end, adding to the drama. Cliché, but Overwatch isn’t a novel.