Hello,

I'd like to present two points, one for lore accuracy, and one against.
1) For lore accuracy
There is a reason why Space Marines have less customization, if all the loyalist marines we are playing are Primaris marines. HOWEVER to keep lore accuracy (as the devs officially stated on Steam the game is 100% lore accurate), Primaris marines should be BIGGER than Chaos marines. Also keeping it lore accurate, Primaris marines are using older helmets sometimes.

2) Against lore accuracy

If you want to keep your "vision" for the game and keep Chaos Marines as big as Primaris marines - not lore accurate - then there's absolutely NO reason whatsoever to not add all the other space marine armor variants, helmet included. 

3) Final word on that, no, I don't think it's unreasonable to want for that, as the entire season pass and all DLC's are purely... Well, colors. That you have to GRIND to unlock. Adding DLC color schemes as they are in the game and planned out is something an intern can do in a Blender, set up materials properly, put on properly scaled chapter mark with alpha channel. This game really needs more model customization.