Hmm. Unless they do some genius here, we're going to get overhyped muscle-bound madmen. Basically think of every bad war movie you can, and then add LAZARS.
The Space Marines are an untouchable force, the classic "legion", trying to characterise a unit of them, hell, even trying to flesh out a commander would be a bad move unless his character has a certain self-hating evil about their person (think the Captain from Pan's Labyrinth). They could very easily make this movie espescially bad if the commander suddenly has a touch of conscience and thinks "GENOCIDE IS BADZ" in a desperate bid to make his character likeable and human.
What this movie needs is a point to show the cruelty, depravity and madness of the universe it is trying to represent. Bonus points if it is told from an alien perspective to make humanity seem like the evil invading force. What would be a good move is if it was told from the perspective of the Imperial Guard on having to work with the Space Marines, since the majority of them are abused, are cannon-fodder and have bleak futures.