The funny thing is, there's a much better IP for this kind of story. StarCraft is basically SpaceMarines vs. Tyranids vs Eldar, but it threw out all of the grimdark and turned it into something much more in line with contemporary movies.
WH40k really defines itself by the grimdark, over the top machismo and grey on grey morality. It doesn't actually take itself serious but is very good at keeping a straight face. Keep that and it will not work well with new viewers. Throw it out and fans will most likely be unhappy.