Nob:
Probably a warrior class, with "shout" type abilities to boost morale (if in the game), cause more damage, etc.
Mekboy:
Engineer type, likely the "destruction" version of a techpriest. Gadgets, mines, turrets, maybe some "repair vehicle" abilities.
I hate classes like these.
Runt Herda:
We saw this class in WAR in the form of Squig Herders. This'll be a pet class, with likely more than one pet (five or so?).
I hate classes like these too, but if the grots/gretchin have personalities, it'll be forgiven.
Guns might be that way... let's go the other way...Mad Dok:
Healer class. If we are to trust the "everyone is a damage dealer" line THQ/Vigil gave us, this class will have an emphasis on buffs and debuffs. My creative juices aren't flowing today, so I'm not coming up with much they can do and still fit the lore. Ideas?