If Russia was added but not Belgium I would personally find the nearest brick and beat a random homeless guy with it. Russia quit halfway through the war because of the Bolshevik revolution - but the country who gave the allies an advantage in the war and fought from start to finish gets nothing? That wouldn't make sense.
While personally I would want Belgium in the game, I'm 99% sure that after they add the UK they'll add Austria so that it represents both sides equally - that being said, if they add a 5th faction, it should be Belgium >.>