The conflict scope of 40k really doesn’t lend itself well to a total war game, how often would more than 2 or 3 factions fight over something the size of a single planet?, trying to involve 5 or 6 and not have to build a campaign map that involves several solar systems is just a scale that would swamp this game (I thought Empire was border line too big).
That in addition to the fact that bar major conflicts the combat is (normally) carried out by smaller yet far more destructive squads and war machines, no, stick to historical and ‘fantasy’ style combat, modern/future warfare for me just would not work as well.
Maybe something akin to Close Combat would lend itself better to the squad sized aspect of 40k but add the tactical aspect that Dawn of War misses out on.