Grey Knights?

It would be cool, but made no sense lorewise ... GKs use completely different loadouts/weapons from regular spacemarines. On top of that each of them is well above average psyker. None of the classes currently in the game is utilized by GKs. So adding them would be only cosmetical and breaking the lore framework (on the other hand, game already broke that, by allowing loyalist characters wear traitor insignia ... which is beyond weird in my eyes) and that kind of beats the purpose.