Yeah, i was just joking earlier.
Write about whatever motivates you. You can write a bad ass metal song without cliche themes.
This is just my opinion, but i've always felt that metal for the most part is 90% musically driven and 10% vocally. Most of the time your locked into the riff, not what the singer is singing about (or if you can even understand them half the time).
Don't throw out any ideas lyrically because it might not be "metal" enough.
Hell, Alexi from Children Of Bodom could be singing about his new kitten for all i know, it's still baddass stuff.
Just go for it man