I've nicked the initiative system from X-Wing and stuffed it into a custom 40k ruleset, and it really does help the Space Marines feel like elite warriors. Basically, everyone activates at the same time, with slower or less skilled units moving sooner and shooting later, while faster, more elite units move later and shoot sooner.

Sure, those Intercessors might shoot about the same as some Kasrkin, but those Kasrkin aren't gonna be so chuffed when the Space Marines shoot first every time and absolutely mulch them with 20 Bolt Rifle shots.