A Warhammer 40k series is coming. For longtime fans of the franchise, this should be a pretty exciting prospect given it will be the first mainstream adaptation beyond online cinematics posted by Games Workshop. While the whole thing started out as a tabletop game, it has spawned countless books, lots of art, and even a few popular video games.
The one thing it has never had though is a television show of expansive scale backed by someone like Amazon. What's more, is that real fan Henry Cavill is set to be an executive producer as well as a star in the show. It seems pretty obvious that the Space Marines are going to be at the forefront of this thing given that they are the poster children for the entire franchise. The question people should really be asking though is which of the marine factions is going to be followed.
There's certainly no shortage of choices here with there being nine Loyalist legions to choose from with far more sub-factions therein. Warhammer 40k is an expansive enough universe that you can even write a single episode about a group of individuals, and it won't actually affect the larger picture given how ginormous the greater universe is.
Blood Angels
We'll start the list with Cavill's personal favorites, the Blood Angels. A chapter full of vampire marines who all end up going crazy at some point due to what's known as black rage. Their primarch (leader of their legion) was slain in the Horus Heresy and every marine in the legion is inflicted with sharing some of the pain felt in this death. As they grow older, they experience more and more frequent nightmares of their dear leader being killed which slowly drives them to an insanity where they feel every enemy of the immortal emperor is responsible for the death of their primarch.
They're one of the bigger chapters out there and have a particular focus on close-quarter combat as a means to stave off their bloodlust. There are a lot of interesting stories you could tell about them as the concept of nightmares means they all are battling against a terminal disease. Experiencing worse nightmares as an old captain ages may prove to be an interesting storyline where you know your hero is doomed. Then the question becomes, how can he die in the most epic fashion imaginable?
Black Templars
A fan favorite for sure, the Black Templars are an offshoot of the Imperial Fists chapter aptly named due to their dedication to going on the longest crusade ever seen for the glory of the immortal emperor of the Imperium. Like their parent chapter the Fists, they don't actually have a homeworld opting instead for a naval fleet along with outposts on whatever world they conquer to stem the tide of "heresy." Warfare has religious overtones in the 40k universe with skulls of the enemy used as incense boats during ceremonies. Having the Templars featured really makes the connection easy to see and would be a good move to appease fans and carries a lot of possibilities when the chapter isn't tied down to any one place.
Salamanders
Warhammer 40k is a dark universe. Life is cheap and Space Marines can barely be called the good guys as most chapters won't hesitate to cleanse and purge even their fellow Imperium soldiers based on accusations of heresy. This can understandably make your main characters a little difficult to sympathize with.
The most sympathetic one among all of them though would be the 18th legion known as the Salamander chapter. While they are no less devoted to the cause, they will at least try to save civilians before they purge a town full of Eldar heretics. They are unique in that they don't believe in innocents dying so that humanity can be preserved. Other chapters would blow up an entire planet just to cleanse it saying that the ends justified the means which may not play very well to general audiences. But the Salamanders are a chance to characterize a small unit of soldiers trying to be kind as the dark universe allows for.
Ultramarines
The safest choice here, there's the most lore on Ultramarines, and there's certainly the most to work with here as they have a hold on some important supply lines in the Imperium. While disliked by some fans, if Space Marines are the poster child for Warhammer 40k, Ultramarines are the favorite child by a large margin. There's so much lore to base any television series arc on that you'd find it dizzying to choose just one. The other opportunity here would be to branch out your series with some other politics unrelated to the front lines of war. There's a lot of bureaucracy in the Ultramar sub-sector, which spans nine-star systems and eleven worlds thus giving license to plenty of schemers, not on the front lines. They're unlike other marines in that they actually have a feudal government system that protects and recruits citizens for the Imperium of Man.
This would be the easiest to really hone in on, as the writers could then easily have a single platoon of Ultramarines to focus on while weaving in a larger story on the same planet. It could be as simple as having the Chaos God's servants invade a planet while Ultramarines root out the cause of the problem unaware that they're being ordered to their doom by the corrupted government. This show is only the start; it's the testing ground to see if the concept can work. Whichever one Amazon ends up spending time with, it'll need to be suitably brutal to fit in with the rest of the 40k universe.