The classic X-COM games rely heavily on procedural content. There's only a small set of simple alien missions, but I think there are three factors that let it s

The classic X-COM games rely heavily on procedural content. There's only a small set of simple alien missions, but I think there are three factors that let it succeed: 1. Harsh consequences. You can't ignore the aliens. Any random alien mission might be the one that makes some country withdraw funding. It might be setting up a tactical battle that's too difficult for you to fight. The alien detection and interception mini-game always has strategic consequences. 2. Hidden information. You start with very limited view of what the aliens are doing. Even when you've researched the best radar you can't cover the enter globe. This makes it easy to imagine they're doing something more complicated than they really are. It's like the argument that text adventure games are more immersive than graphical ones because the images are only limited by your imagination. 3. Gradually unlocked story. The games have a research tree, and as you retrieve alien bodies and technology from random battles you unlock more things to research. Many of these research items serve only to explain the story. They do nothing to help you win but the first time you play they're some of the most interesting things to research. It gives you a real feeling of progress beyond just getting better weapons. Many games feature the first two factors, eg. roguelikes, but I think the addition of the gradually unlocked story is what makes the game truly great. It combines the mimesis and diegesis that Jimmy Maher talks about in a way that makes both of them better. Also, the games are available on GOG, but I strongly recommend using the Free Software engine reimplementation OpenXCOM if you're going to play them, because it fixes a lot of bugs and interface inconveniences.