The Irony is, if GW ever tried to make a 40K Movie based on one of the bigger Campaigns, take any Black Crusade, or one of the 3 Wars for Armageddon, Bay would be quite enough. There is no real deep Plot to deal with, its a massive fucking Battle, in the case of a Black Crusade, even across several bloody Segmentums (for the non-40k knowledgeable, a Segmentum is a part of the Universe, there are a total of 5. Still confused? Look it up).
What Character Development and Plan/Lore there is cant really be fucked up that badly, they mainpoint is as said, explosions, shooting and a shitload of guys in Power Armor.
Bay is a terrible Director, i agree there but seriously? If you take one of the major Conflicts which does not involve all that much subtlety, he cant really fuck things up that much. Which is probably also why he wouldnt do it. I believe he takes some perverse pleasure out of taking a Franchise and running it gleefully into the ground.
Anyhow, GW wouldnt let him do it, or let anyone do it. GW will publicly crucify you if you get the "go ahead" from them and then dont stick to Canon. We all know what happened to People who mentioned Squa-