There wont be any campaign.
On Topic:
Yes mixed allied factions is a horrible in my opinion. Ostheer and the OKW fighting together can make sense but will probably OP if US/Sov teams are not possible so mixed german factions will have to be taken out as well.
Relic stated a lot of times that they are trying to create a somewhat authentic (not necessarily realistic) experience. "Immersion" has always been part of Relics idea for the game (at least I did read that in an interview a while ago as far as I remember). This will utterly destroy that immersion.
The solution is simple but will split the playerbase (and will have an impact on the leaderboards): Let players choose if they want to be matched up against mixed teams or not.
For me there are things that im okay with concerning authenticity:
- Rare vehicles roaming the battlefield (Ostwinds)
- US forces fighting on soviet maps/soviet forces fighting on western front maps (though some kind of mechanic that replaces t34 wrecks with sherman wrecks and vice versa depending on which factions are involved would certainly be great)
-Ostheer fighting Americans/Soviets fighting the OKW
-Ostheer/OKW fighting Americans OR soviets (though probably too unbalanced to allow for automatch)
-Obviously the unrealistic gameplay mechanics
But having IS2 tanks fighting alongside US Airborne rangers? That is too much imo. I dont need a perfectly authentic game but something that I feel has still something to do with the actual WW2.
Honestly, i don't think soviets fighting the new OKW ruins immersion. I mean they are just germans, however sovs+ americans does.