You know what you should play: DOOM
$10 for Ultimate DOOM, DOOM II, and Final DOOM will get you thousands of fan-made levels and missions beyond your wildest dreams. In fact, DOOM is one of , possibly THE greatest, PC games ever made!
Here's a link to prove my point.
And if it looks too dated, download JDOOM and stock up with high-res models and textures. You can also get MP3 music and sound for the game as well.