40K game changers has a great new interview with Rick Priestley here, outlining the origins of 40k. There's some fun commentary here.
Initially it covers the point in the mid-1980s when they were selling board games on the high street, rpgs, they had a license for D&D, Judge Dredd etc. The amalgation of GW and Citadel Miniatures, and the reason they ended up as Games Workshop and not Citadel (shop signs are expensive and they already had 10 shops). Thus the gig became about games being the medium of selling miniatures, and Rick Priestley happened to be the guy who had access to his mum's typewriter. He talks about how WFB had become the breadwinner, selling tons of minis off the back off that way in advance of all the licensing and RPGs. Priestley had initially written Rogue Trader, which was going to be a spaceships-and-trading games. There was an ad for Rogue Trader with a pic of a spaceship and a promotional spiel in a Warhammer compendium, and so people kept asking about it.
With sci-fi elements like the Slann already existing in Warhammer, he came up with a sci-fi variant than then became Rogue Trader as the game name and promotional stuff was already around. Spaceship combat was still a part of it, and from that space marines appeared. Sci-fi at the time (despite Star Wars) had the rep of not selling, and so they came up with weapon packs so that fantasy models could be converted into sci-fi ones. Cue Eldar, Orks etc as they kitbashed models to test the rules...
He mentions "there was nothing gothic about a tank made out of a deodorant stick. I had talked about 'gothic' only as a mindset, but if you tell that to artists, they start drawing cathedrals." Dune, 2000AD (due to their connections with the license), Michael Moorcock and the age of sail all cited as influences. Also the idea of the heroes not necessarily being 'good guys' (Aliens had just come out). Apparently Americans didn't like this.
Tyranids- Priestley came up with them, and Brian Ansell reimagined them, setting the 'the shadow in the warp moving at sub-light speed'. To which Priestley laughed and says 'the galaxy is 30,000 light years across, they really wouldn't be much of a threat...' Dark Angels, Lion El Jonson and naming characters: 'How many people at the time were into late Victorian poetry- now you can just look it up.'
Same for the idea of mystery and rumours, which the internet, by cataloguing it all, has removed slightly. He doesn't read the Black Library books, as it's too different from his own concept now.
Warhammer runs on d6s because at the time every UK kid had d6s in a monopoly set or whatever, as opposed to the d10s, d20s etc used by other specialist tabletop wargames at the time.
Much more in the interview.