Game: Grand Theft Auto IV
Guess what this one does. A bowling league for Roman? Cars that drive themselves? A character who appears to tell Niko “You have $30,000 in your pocket, you don't need to goon for assholes” after Act 2? No, of course not. These guns put a little reality back into the cartoon that is GTA. The missions weren't written with that in mind, obviously, but there's nothing stopping you from giving it a shot. Worst case: murdering random civilians on the street is much quicker, easier and more satisfying. At least until the cops show up to spoil the fun. Range, accuracy, damage, ammo and fire rate are all covered, though be warned that you shouldn't expect perfect accuracy from your upgraded hardware. This is GTA after all. Realism is not baked into its combat engine.
The Long War
Game: XCOM: Enemy Unknown
You're looking at eight soldier classes, many more missions, invaders as focused on upgrades as your own science team, and a much longer path to victory. Research is slow, not least to make early weapon upgrades more useful, while the aliens are constantly getting more powerful. Their ships are better, their terror missions are more regular, and more of them show up for battle. In exchange, you get to field more Interceptors, the council is easier to appease, and the ETs don't cheat as much.
Game: Far Cry 3
Ziggy makes Rook Island a more natural place, removing mission requirements for skills, cutting some of the easier ways to earn XP, increasing spawn rates to make the island busier, and throwing away the magic mini-map in favour of a compass. The second island is also unlocked from the start. Smaller changes include randomised ammo from dropped weapons, being able to climb hills that you should realistically be able to, and wingsuit abilities made available earlier to get more out of them.
Minecraft has a Survival mode, but it's not desperately challenging. Terrafirmacraft takes it seriously, with hunger and thirst that must be dealt with at all times, and key elements added such as the need to construct support beams while mining to prevent cave-ins, and a seasonal cycle that determines whether or not trees will produce fruit. Many more features are to be added, but there's enough here already to make survival about much more than throwing together a Creeper-proof fort.