Yeah, have to agree with Alexel above. Just about anything has been made before already, if we want to argue that way, and 40k by itself is proof that simply rehashing and giving it a fresh paintjob works out. The vast, vast majority of TV series and movies that we get to watch these days have been told before in some manner or another; what makes them interesting is the setting and the characters, which tend to give these stories a new twist or just make them more enjoyable to watch because people dig the background and atmosphere of the world.
Where 40k would excel is hamming up the extreme and embracing the utterly ludicrous. The comedic and satirical aspects, once so integral to the setting, would make great movies. Unfortunately, that ship has sailed, as everything must be more grimdark and Marvel has released Guardians of the 40keks before the idea even occurred to GW. They missed out bigtime on the comedy aspect and not taking themselves so seriously, and as GotG proved, you can still tell a good, serious story with a lot of laughs. Sadly, Caiphas Cain and co. who would work in this format are the unloved stepchildren.
This I also have to disagree with, vehemently, although I'm sure it's just a matter of personal preferences. To me, turning it into a silly comedy would do the setting just as much of a disservice as the current trend of silly ÜBEREPIC does. Proper Grimdark would work like a period drama a la Name of the Rose or Black Death, where crazy conventions that would well qualify as satirical are presented in such a straight-faced manner that they are as creepy as they ought to be if the setting were a real world with real people reacting in realistic manners to whatever they face.
At the risk of Godwinning the thread, I'm sure all of us have seen how easy it is to do comedy with Nazis, but obviously that doesn't mean that they were good for a laugh when people were actually being at risk of being killed or tortured by them, and it really isn't hard to convey this in a movie; we have multiple examples of this, too.
The only question is whether you'd prefer a show that lets you immerse in an actual living, breathing world, or if you just want it to give you a good laugh. Both approaches are viable, but I daresay that a lot of 40k's potential would be wasted if you'd stick with the latter. Just as if real life war movies were limited to stuff like Blackadder and Hogan's Heroes, rather than giving us Apocalypse Now or Full Metal Jacket.