Odds are it will be: Tactical Marine, Devestator, Imperial Trooper, and Possibly Guardsman Sargent?
Especially seeing as Grey Knights (Demon hunters) are not uber common (Like they would be if they were playable.) since they are Inquisitorial (And as such, only report to the Inquisitors, nobody else can really tell them what to do apart from the higher ranks of Marine commanders, since Guard commanders are more or less controlling meatshields.)
Grey knights appeared in Dawn of War, they are uncommon in 40k, but they tick lots of balance boxes. Close combat, Psycher, Ranged.
Spesh as Eldar will obviously have at least on Psycher, same with Chaos i'd of though. Orks have the weird boyz.
Just going on MMO building, theres normally an equivilant set of classes for all groups.
Predicting.
Imperium:
Space Marine - To go - Assault, Tactical or devestaor
Guardsman - To go, Kaskirn, Heavy or a sergent class for support
"A Psycher" (Librarian, Grey knight) - To go Assault, Support or ranged
Eldar:
Warlock - to go Support, Assault or Ranged
Aspect warrior (not specified until ?talents?) - Dire avenger, Banshee or Dark reaper
Another - possibly scout class or something.
Chaos:
Chaos Marine - Assault, Tactical or devestaor
Khorne Bezerker - Kill, Kill or Kill!
Sorcerer - Assault, Support, Ranged
Orkz:
Boy - assault, ranged, loota
Nob - assault, stomp or bash edz!
Weird boy - Support, ranged, assault
Prediction isn't overly balanced and there could be more than three classes, just follow the trend of assault, ranged and support and it can't be far off.
RELEASE MORE INFO ALREADY! AAAARRRRGGGHHH!