honestly, i would really prefer they had not included chaos and loyalist marines as their own factions. include traitor guard and imperial guard, and then make regular orks the normal class, and make guardians/storm guardians the regular class for eldar. space marines could be elite units for imperial guard, along with ogryns, sanctioned psykers, and whatnot. aspect warriors and warlocks would be elites for eldar, nobs of any kind would be elite units for the orks, and for chaos you would get chaos marines, possessed guardsmen, and probably some other stuff for elites. thus you can afford to make marines a good deal more rare and powerful--or, more accurately, you would have something to compare them to that they just are vastly superior to.