If people like the idea, I'll write it all the way through probably.
So basically there was a strong WAAAGH! that hit a proto-Hive world (so it had a wide selection of people with different talents), and so a couple successor chapters (along with a squad of Ultramarines) as well as the Imperial Navy (carrying quite a few guardsmen and such) assisted in defending the world.
However the WAAAGH! was far too strong and the planet was already too ravaged, and so the Navy (with civilian ships, eager to run) entered the Warp in an emergency jump, which accidentally brought Ork forces with them.
But once they left transit, they showed up in the middle of nowhere, with Ork forces scattered and likely closing in. In their mad dash towards the nearest Imperial stronghold, they found nothing but scraps, and so a small team of ten Astartes landed on the planet. Surveys suggested...
...the planet's Imperial presence was wiped out 6,000 years ago.
So that would be the intro couple chapters, it's basically BSG but time traveling 40k, with the plot points that comes from that.