Space Marines can be developed characters, if used properly. The script will need to be something incredible, encapsulating the minds of non-humans trapped in a cycle of war and death, summing up a warped psychology in the face of a warped galaxy. The gold would be in creating the Horus Heresy trilogy, cutting away much dead flesh to get to the bones of that narrative, being careful not to cram too much into each film and letting the characters carry the awesome. Tell it from the point of view of humans if you must, interspersed with space marines and use the juxtaposition to drive the themes of the film (and it must have themes, it must have a point) and the mounting tension.
Begin with a joyous and over-the-top view of war, of men running happily into battle like it was 300 all over again, and then drag it downward, have battle become more dreary, watch marines become sullen wrecks or psychotic berserkers as the constant war takes its toll. Show them to be human, deep down, in their minds, even though they are living weapons. Strip the romance from the combat over the course of the film, watching Horus decline as you do, cut the awesome and let reality seep in as men from the squad disappear one by one, for some reason I get the urge to abuse a montage (perhaps at the start of the film? To set the scene?). Take the illusion and mystery and fun of this wide and wild universe and over the course of a film you drag it down into the mud and show it for the hell it is, hold the bare marines up the audience and say 'Here is your superman, a weapon, brought low by all you smile at'. Strip illusions, make 40K darker. Don't make it cool, don't make it awesome, not by the end.
Challenge the perceptions of what constitutes humanity, make a film about what defines us as people. Make a film about flaws and weakness, shown against the backdrop of a supposedly perfect man, watch him come apart.
And then add Bolters.
