Well going on the fact they have stated it is definately a MMO, I'd expect Space Marines to be a starting character, and probably the main Tank for what ever faction it belongs to. As to balnce, well they'll just do it, bit like how in Warhammer you Have Arch Mages, Warrior Priests and other hero level characters from the tabletop standing alongside troop types and thinking hows this fella so hard. Also in 40k tbh the only time you'd roll your eyes is if a Marine is done over by a guardsman. Now odds are Guard and Marines will be in the same faction, so it'll only be guard v's Chaos marines anyway. Plus depending on their class choices for Guard it might be a non issue anyway. Say if the Guard choices where Commissar, psyker and Ogryn, you'd not even worry about it. Of course this is all speculation, but its something to consider when you think what WaR have done.
On a side note I did see a Dev state that all races considered important by the fans would be there, which is an odd statement in my view. As on one hand he hasn't said they'll all be in there, but all races important to the fans would be all of them. So I'm guessing thats a roundabout way for saying Marine, Guard, Ork, Chaos, Eldar and probably Dark Eldar will be in, probably teamed off into two factions.
I'm defiantely going to watch this from afar as they get stuff together mind you. Of course the death rate for MMO's in dev is pretty high, so who knows if it'll get to launch, I hope so, also be interested to see if any of my predictions are near the money as well.
But I do hope this game makes it, by the time its launched I'll probably be ready for something new hopefully after 3-4yrs of playing WaR.