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.
The flip side of the coin is you can easily cherry pick the strongest options from a variety of forces to build a very individually packaged pasteurized processed force. I'm sure this is the 'Merican Way.
My latest commission is a soup detachment that might grow into some small armies or army specific detachments at some point. Right now my client isn't so concerned with gaining access to stratagems as he is warlord traits and relics. (Notably the Command Point Factory guard have access to)
Five scout squads, an Imperial Commander, Primaris Psyker and a Custodes Jet Bike Captain round out this force.
The scouts forward deployment allows him to block off portions of the table to prevent deep striking. They also can claim objectives and be a general annoyance for most opponents. Being as these with be mostly partnered with the Ad-mech Robots, they'll also provide a screen.
The Imperial Commander is there solely for the Relic and Warlord trait that allows you to gain command points whenever you or your opponent spend them. This is by far the most annoying part of the guard codex and being able to shoehorn it into a Soup list makes it even more annoying (that said it's pretty strong and worth taking if you need a pool of Command Points). This guy will most likely find a place to hide and avoid being killed to deny the opponent Slay the Warlord.
A Primaris Psyker gives access to a cheap smite or the power that daisy chains mortal wounds on a good roll. The ability to Deny the Witch is also pretty solid for a cheap HQ to fill out the required slots.
Finally, there's a Custodes Watch Captain on a Jetbike. This guy is a beast and looks really cool. Basically it will pump out a bunch of firepower and distract from the Robots pumping out a ridiculous number of shots.