My idea of a 40k movie would be a Trilogy of the three great recent wars that have been in fluff and become campaigns. Armageddon, The Gothic War and the Eye of Terror.
Movie 1: Warhammer 40,000 - Armageddon
Tag line: "In the grim darkness of the future, there is only war." (duh!)
Synopsis: Third War of Armageddon, story revolves around Commissar Yarrick and General Kurov (Imperial Guard perspective), Admiral Parol (Imperial Navy), Herman von Strab (he will provide the 'inside' view of the Orks through human eyes), and High Marshal Helbrecht (Black Templars). The film will switch around between the commanders like in "A Bridge Too Far." It would show, especially to people who don't know 40k, the epic struggle of humanity to survive in a dark future full of danger, death and war.
Movie 2: Warhammer 40,000 - The Gothic War
Tag line: "The war continues among the stars."
Note: I don't really play BFG, but I've read some of the fluff from the EoT book.
Synopsis: Abbadon and his Chaos fleet has entered Imperial space. Story will switch back and forth between the Imperial commander (who?), the Eldar fleet commander (who'd provide the Eldar perspective) and Abbadon (Chaos Marine perspective). The movie will show how terrifying Chaos is and introduce the ancient Eldar race, also to serve as a prequel to the third movie... 'Course it'll show really cool space battles, since Armageddon was mostly a 'land war.'
Movie 3: Warhammer 40,000 - The Eye of Terror.
Tag line: "Let the galaxy burn." (Where have I heard that one before?)
Synopsis: After the Gothic War, Abbadon has returned and this time he leads his 13th Dark Crusade. This time it'll be total war with space and land battles involving Imperial forces, Chaos armies, Eldar warhosts and the other races involved. It'll also show the Tau taking advantage of the war took invade Imperial worlds. The focus would be around the world of Cadia and her defenders and the Chaos invaders. And as we all know, in the end Cadia will hold, but by herself...
Wel, those are my ideas anyway...