A image posted to the For Honor subreddit indicates that the first two DLC heroes to join the For Honor lineup could be the Centurion, probably for the Knight faction, and the Ninja, who will obviously run with the Samurai. It's entirely unofficial, and there's a bit of placeholder text in the image that looks a little dubious, but on the whole I'd say it looks pretty believable, even if it's not yet clear how the new heroes will fit in.
The Orochi class already employs some ninja-style tactics, while the Nobushi would appear to have the light-and-fast angle locked up, so the Ninja's place in the current hierarchy, without the addition of new mechanics or abilities, is a little hard to nail down. GamesRadar noticed that the "Stealth" feat is selected in the image, which keeps the character's information hidden on the screen and mini-map to make it easier to sneak around and gank the unwary—A very ninja-like style of play, yes, but how he'll stand out from other characters capable of the same thing isn't at all clear.
The Centurion is clearly Roman, and while he will presumably join up as part of the Knights, some Redditors believe that he could actually be the first member of a neutral "mercenary" group employable by all three factions. Regardless of how that works out, he looks like he'll be a pretty straightforward frontline soldier, but again, the mystery is how he'll be meaningfully differentiated from other all-purpose sluggers.
It could be that somebody's just playing games here, but the two heroes do bear a remarkable resemblance to the pair standing at the front of the season pass image. I've emailed Ubisoft to ask if the leak is accurate, and will update if I receive a reply.