Producer here with minimal to no-knowledge of the source material, aka the best person for the job.
Alright, here's what we do:
Imperial Guard, they're more approachable and human, right? Can be played by pretty actors and women? They're pretty much the colonial marines from Aliens?
We set it up as a team of soldiers from the Imperial Guard being sent to some whatever base in a far off planet to investigate a distress call. Maybe the planet was researching something to help them in the war.
The team is pretty casual, there's some banter and quips to make them likeable. Maybe 3 guys, divided in the usual leader/sarcastic guy/meathead and 2 women, one is the no-nonsense nanny and the other is the quriky nerdy one.
They meet some scientists, fight some Tyranids first because they're easy mooks that can lead to a quick and good action scene.
Next, an ork spaceship crashlands on the planet, they're looking for whatever MacGuffin is held in the planet. Why orks? Simple, they're green, silly, have accents, audiences will eat them up, we could sell a lot of merch with them.
Then we get a Space Marine, he's sent after the team requests help. He's stoic and noble, he's gonna be our serious side for the film.
So now we can start act 3. Meathead dies while a bunch of shit is happening.
Scientist has been taking them to the center of the base, and it's then revealed he was actually evil the whole time and we get Chaos in, because space-slugs and dumb green guys don't make serious stakes, demons do.
Then it's a run for the macguffin, Space Marine destroys it and has a final fight with a demon-scientist or whatever.
Everyone dies.
Leader guy sacrifices himself to save quirky girl, saying he always loved her.
Movie ends with Space Marine and girl survivor on a ship back, she's all sad and shit, the Space Marine vouches to help her further investigating the macguffin and we set up the sequel.
With a good marketing campaign we can get at least 600M.