David Weber's Dahak Trilogy. Well, the first two books, really, as the third one is a different story from a generation later.
It has everything for an epic game. It could have two distinct halves: A Walking Dead type adventure mode and ridiculously high-scale and high-tech space battles! I mean, your flagship would be THE FRICKIN' MOON! And Dahak (aka. the sentient ship that is disguising itself as the moon) is not even the biggest of the Fourth Imperium's planetoid-type warships! You don't really get any more high-scale than that!
Even more importantly, the story has everything you can ask for: a relatable audience surrogate protagonist, great mysteries, awe-inspiring scale and technology, great characters, a romance-subplot that actually reaches its apext in the middle portion so it would take a back-seat to the awesome final battles, and the story has so many "OMG!" moments I didn't even bother to count...