Why are.you imagining costuming is going to be a problem? Big coats, themed future outfits, and sci-fi accessories isn't exactly breaking new ground.Interesting. I'd watch it. Prepare for the torrent of forum wars because someone in costuming had to make a sacrifice on the size of shoulderpads to let their actors actually see.
Live action is probably the big millstone to me though. Making the stuff they wear in games/art/etc. wearable irl for a movie is going to be tricky. I know there's some fantastic cosplay out there but the proportions on it can look weird if they try to be faithful and practicalities are not exactly a 40k thing.