1. Leveling Systems
The great thing about games now is that everything is progression-based, and I think that's a great system to implement in shooters. Having a goal to reach in terms of unlocking guns, skill points, or other shiny things allow games to break away from their stagnant game modes of "Kill X amount of enemies" and rinse and repeat. By having something to actually look forward to when you're out counting how many headshots you've accumulated, it creates a stronger game loyalty. People are addicted to progression, and the more options you can provide for people, the longer they'll play the game. If you think there are any other game-changing mechanics that have made FPS games better, or feel like one of these don't belong, please let us know in the comments below.