OST Mashup- What instrumental song could you see being played in an anime?

Sounded fun in my head- what instrumental song (no vocals, simply background music) could you see playing in a certain anime? Kind of like you choosing a piece that would appear in an OST to a certain scene.

I could see this song being played in the background of Death Note, maybe as somebody's character theme that came on whenever they were on screen. I feel like it just kind of holds that sort of air to it. Not sure whose theme it would be, though. xD;;

Go ahead and post your own~