Warhammer 40K is possibly the best FPS material I've ever heard of, it's true that it would take a developer with balls of steel to tackle it, but that's what would make it so great - you'd be left with either a total flop you could just forget about or something mind blowing. The possibilities are endless, you could end up with an Alien vs Predator experience where each race is playable and all play differently, multi-player would be a blast.
You could end up with a bowel emptying horror - a lone guardsman trying to get out of a dark city over-run by Tyranids, with smaller Tyrandis scuttling over buildings and dragging people away, giant beasts roaming the streets, and you picking your battles trying to get to the last drop ship that's about to leave (Similar to the marine sections in Aliens vs Predator).
You could have amazing action - playing as a space marine riding a Drop-pod down into a city being attacked by Orcs, charging in with a bloody chain blade and bolter gun, spraying blood and gore, destroying anti-aircraft guns so that landing craft can bring down reinforcements (similar to Gears of War).
You could be part of a Tau Pathfinder squad, jump jetting with your team mates across the tops of smashed buildings to set up a snipers nest to reinforce a position that's about to come under attack.
You'd end up with a brutal multi-faceted game, with dozens of unique and interesting enemies, a cinematic experience rate R. Then when it comes to multi player, you could have coop, or battlefield style maps, or multi-species death match (Orcs rushing forwards with axes, marines blasting away with bolters, Tyranids scaling the walls). The beauty of WH40K is it's such a vast universe you don't have to cover everything, you can pick and chose and it lends itself to what you're doing.