This is a very random out of the box idea, but was thinking it could have been interesting if it was revealed that all the Sith troopers were the original clone trooper template from the prequels, Temura Morrison. Probably no way to properly execute it in film other than perhaps giving some of them the clone distinctive audio during the final battle and perhaps finding a way to mention a clone army.
I found a sound bank of Republic commando sound clips. I did add some more of the grunting and yelling sounds at certain points. It isn’t obvious, but those who recognize the sounds/voice would be able to pick it up.