I'm hard pressed to list a video game that wouldn't benefit from modifiers like XCOM: Enemy Unknown's second wave options. These are advanced gameplay modifiers you can enable to increase challenge or add replayability - similar to Skulls in the Halo series. It's shocking how a few tweaks to XCOM's turn-based strategy formula completely upend your strategy against the alien invasion. Some ramp up the difficulty, while others make the game more random, and together they force you to reevaluate your playstyle in new and interesting ways.
My personal favorites change how your soldiers grow. In boring-old-regular XCOM, rookies are basically all carbon copies of each other, with identical stats and level progression. But in party-time-second-wave XCOM, you can enable Not Created Equal and Hidden Potential to mix up your rookie's starting stats and how those stats change between levels. This may sound like a minor detail, but in a game where a few percentage points are the difference between brave and grave, messing with stats like this can really surprise you.
Other players I know swear by Aiming Angles (bonus accuracy if you're near flanking) and Red Fog (accuracy penalty if you're injured) and won't play without them. Options like these crank up the heat inside the alien-infested pressure cooker that is XCOM by rewarding smart play and punishing mistakes. Instead of simply choosing between the static easy/medium/hard options, these second wave selections tailor the difficulty to your liking, creating a better experience overall.
XCOM: Enemy Unknown isn't the only game to do something like this; Halo has its Skulls, Mortal Kombat has its Kombat Kodes. But games that offer this sort of remixing remain scant. I'm sure there's something you wish you could change or tweak about your favorite game. For me, when I fire up my semiannual playthrough of Pokemon LeafGreen, I wouldn't mind mixing up my starters, or pumping up the difficulty. These are simple requests, ones that should not be resigned to the labor of ROM hackers or other modders.
Time and again we see game developers rolling out these giant DLC packages filled with fancy new guns, levels, characters, and more (XCOM included). And while these additions add some longevity to their base games, I believe an equal amount of enjoyment can be derived simply by letting players tweak the game itself.
For more XCOM be sure to check out our exhaustive XCOM 2 preview, complete with a livestream with Firaxis Games.