If you go back and watch all the videos, panels and look at all the artwork you will see well over 20 unannounced characters that could possibly be added to the game. Here is a look at just 18 of the characters not announced (yet):
http://i.imgur.com/FAYNHPi.jpg
Some notes:
1, 2 and 3 are highly likely to be in the game as they are on the games final poster/wallpaper and were hinted at during one of the panels.
8, 9, 10, 11 and possibly even 14 could just be random soldiers thrown into the art and seem like they don't have enough character to make it into the game.
17 could possibly be an early Widowmaker concept.
Mentioned in Trailer:
SoundQuake - Possibly the dude in the green clothes as he has headphones and his "gun" seems to have a subwoofer on it, although chest missiles were mentioned.
Fusionator - Possibly the guy in the hazmat suit.
Doomfist - Possibly retired/deceased, the elder brother could take that call sign as he was seen using his gauntlet in the trailer.