Hear me out. Let's say instead of violent nerd Gordon Freeman wasting alien invaders in City 17, Half-Life 2 was actually about a gritty hardboiled detective fighting corruption in Prohibition-era Chicago. That's the concept behind The Burton Equation, a Half-Life 2 mod you may have missed because it came out on Steam between Christmas and New Year's.
Don't worry, though: Burton does just as much shooting as Gordon Freeman does, only he gets to use a weapon even cooler than the gravity gun: the drum-fed Thompson submachine gun, otherwise known as the Tommy gun. You haven't lived until you've stood in the headlights of a Model T and chewed up a handful of Metro cops with a Tommy gun.
So, what are the Combine doing in Chicago in the 1920s? Don't worry about the finer details of the plot, okay? A crime boss named Fabreen is running the city, and his Combine Metro cops act more like a gang than interdimensional invaders, strolling around the city in fedoras and trench coats. Detective Zachary Burton (that's you) has had enough of Fabreen's corruption, and after a visit from a mysterious dame, decides to do something about it.
Burton's plan is not all that complicated: Half-Life 2 is a shooter, after all, so Burton just starts shooting. In lovely black and white, accompanied by Burton's gritty narration, you make your way through the gloom and rain of gangland Chicago, icing every Combine you see.
But Half-Life 2 isn't just a shooter, it's a sci-fi shooter, and there's a few subtle bits of sci-fi woven into Burton's hardboiled, lead-filled quest for justice. Such as, well, time travel. Fabreen is one slippery gangster, and once you've back him into a corner he starts jumping through time, leading Burton on a merry chase through the trenches of World War 2, the neon-soaked streets of 1980s Miami, and eventually into Gordon Freeman's future where the actual Combine invasion of Earth is under way.
Is it the most logical alternative history I've encountered in games? No. It's not the most ridiculous by a longshot, though, either, and like I said, you get to pick up a Tommy gun and mow down Civil Protection while listening to a detective growl narration. You can untangle The Burton Equation on Steam.
