This page lists or shows all the heroes that have been in some sort of media and who could potentially be a part of Overwatch or included in the game at some point.
The following stills are taken from the animated short Recall in which Winston sends out a recall signal across the globe to all former Overwatch agents.
Seen in far left of "Group 1" shot below, as well as potentially on the left of "Other 4" shot below
Maybe the front man in "Other 3"
Maybe in lower left of "Other 1" shot below
Seen in far right of "Group 1" shot below
Also in center-right of "Group 2" shot below
The following are group shots that show some unknown heroes:
Other 4, the woman on the far right may also be the woman in "Other 3"