Campaign is not the biggest problem if relic decides to make an eastern front expansion. The problem occurs as an eastern front campaign featuring the Red Army and a new arsenal of weapons naturally will be accompanied by a Red Army faction in MP. Therefore Relic has to figure out how they can incorporate this faction without creating major problems regarding the historically accurateness of the game (example US and Soviets fighting together in Normandy).
As some mentioned, they could simply give the players the option of choosing whether they want to play eastern front (Red Army vs a German faction) or the Western Front (US and Brits vs Wehr and PE).
... That made me think about the ToV campaigns. It didn't seem like a PE campaign was incorporated as the Tiger Ace only consisted of Wehr Tigers and the Falaise Pocket mostly consisted of Wehr units along with ACs (first mission only) and Luftwaffe Ground Forces.