An exercise I like to do is taking a song I've written and flip flop the key. This previously took a dark waltz and turned it into a sweet sounding ballad of sorts. Trying to keep the core of the song together while flipping the key is just a fun experience that I can never get enough of.
The Fallout was previously a brutal, heavy song that punched it's way forward. When riffing around, the riffs still seemed to work in a major key so I went with it.