Warhammer 40,000 – One of the coolest (or most annoying) things about 8th Edition 40k is list construction. The Keyword system means you can cherry pick the units you want to build whatever force you want. From a thematic aspect this is very cool as it can represent the disparate forces abandoned to the darkness after the Great Rift ripped the galaxy in two. Which means it's feasible to have several units of marines traveling with the remnants of a guard regiment being led by some group of Imperial Heroes with a Knight backing them up. Or recreating odd pairing that you see in Black Library books all the time. For instance I'm reading Death of Antagonis right now and am tempted to pick up a Canoness to run along side my Imperial forces. It's not super strong but could make for a cool narrative.