Yeah I thought the smoke was never going to clear. Considering its only 1.39 and about two thirds of that is smoke and the outro my interest has still been piqued.I am impressed by the action footages, I was afraid the whole trailer was going to be a bunch of smoke flying around.
My best guess is that they'll probably be fighting orks. Looking at the trailer, those wounds some of the marines sustain are large an messy which indicates to me heavy callibre rounds, which also narrows down the list of possible enemies.As for races I think they're going to keep it as black and white as possible i.e. human versus alien to keep it simple and more accessible for people who might not be familliar with 40k. For that reason I expect to see SM's (obviously) Tyranids and/or Orks as they're motives don't take too long to explain. Maybe a few IG sprinkled in as fodder. I think CSM's and Chaos in general are definitely out of the picture as it opens a can of worms and would take to long for a single movie to explain without completely deadening the action.