In some of the Space marine cut scenes, there are space marines with white helmets, but I never see any of these guys in game. Anyway they could be a skin for killteam? Or even better, make them an upgrade for tacticals at t3 that just buffs their stats? We could have the same for the Orks and Eldar, 'Eavy boyz and Squads. I think that'd make your basic t1 Line units a bit more appealing in the late game.