Anything you bloody well feel like, is the short answer to that.
But surely, I hear you cry, if I do a song about all the kittens and puppies and how they're all so sunny and happy, I'll be laughed at for being a poser of some sort, won't I?
Well, not really, say I, because Wargasm's first demos were called "Rainbows, Flowers, Kittens, and Puppies" and "Satan Stole My Lunch Money." S'there. But anyhow. Here's some rather, well, unexpected, topics that metal bands have done songs about. So sit back, and enjoy, safe in the knowledge that it's not just Satan, heathendom, gorno, drinking, and weird crowings about Odin.
1. Unrequited Love
Examples of this include "The Hardest Part of Letting Go" by Megadeth, and also "The End of This Chapter" by Sonata Arctica. Unfortunately the unrequited love in both these tracks is so unrequited that it crosses the line from pining, through creepy fantasies, and into stalking territory. In fact, "The End of This Chapter" opens with a recording of Tony Kakko making a heavy breathing and creephatted call to a former lover, "oui oui, mon amour, c'est moi!" and explaining how she thought it was impossible but he steals her earring while she's asleep, and so forth. Less creepy is Skyclad and "My Naked I" which is about fantasising from a distance so it's more unrequited lust. "I'd say without a doubt you're the best sex I never had, because in my dreams you wore a necklace white and pearly. Make mine a double bromide on the rocks please, stirred and shaken. Should I wait till it's too late - or come too early?"
2. Warhammer 40,000
Bolt Thrower (with songs like "World Eater," "A Symbol of Eight," "Through the Eye of Terror," "Realm of Chaos," and "Warmaster") are one, and Debauchery (whose work includes "KILL BURN MAIM") amongst others. Also Sabbat's first release was a flexi-disc of a very spiffy song called "Blood For The Blood God" and it was distributed in an issue of White Dwarf in 1987.
Carnivore, the band Pete Steele was in before Type O Negative, on their first album, "Carnivore," had a song called "Carnivore." It's about how rug munching is awesome. "Lick me, she begged, and pulled down my head. I LAAAHHHVE, TO EAT PÜSSEH!!! A taste so fine as sweet April wine, I wouldn't trade for any money."
4. The Byzantine Empire
In 2011, Turisas released an album called "Stand Up and Fight." It's a concept album about being a Varangian mercenary in the Byzantine Empire in the 11th century. In a way it's a sequel to "The Varangian Way" from 2007 which was about going on the dangerous overland journey from Finland to Konstantinopolis to trade.
5. Classic Video Games
By way of example, Gama Bomb and "Final Fight" which is about, yes, Final Fight and includes the opening line of "Korobeiniki" (the Tetris theme) as a solo. Also "Vampire Killer" by Battleroar which appears to be about Castlevania, and "Hell on Earth" and "At the Crack of Doom" by Warbringer which is about Doom - "Trapped in a maze, you must escape. Standing a doorway a demonic shape. Barons of hell spewing green fire." etc. Atop all this is the existence of a band called Powerglove whose first album "Total Pwnage," was a load of metal covers of video game themes.
6. Comic Books
Gama Bomb again with "Return of the Technodrome", "In the Court of General Zod," and so forth; Hospital of Death with "Deadly Wheels of Steel" about Judge Dredd, Iced Earth with "V" (as in V For Vendetta), and of course Bonded By Blood with the Turtles theme thune as a bonus track on one album.
Yes, there is at least one metal song about chess. It is "Kill the King" by Rainbow. Think about it - "Danger! Danger! The queen's about to kill!" - the queen being the most used piece to stage a mating attack with; "Oh no, move me out of harm" because he's in check or a piece under attack; "I'm no pawn, so begone, speed on and on" because pawns are, of course, expendable (well, up to a point) whereas pieces aren't necessarily. Also, killing the king is the objective of chess. Also, RIP Ronnie James Dio.
8. Ancient Egypt
Everything Nile has ever done ever. (Apart from "Kafir!" which is standard religion trashing (Islam, in this case, which is rather unusual) but still awesome.)
9. Biblical Apocrypha, Kabbalah, Hebraic Legendry
Everything The Meads of Asphodel has done ever. (Apart from their slightly iffy covers of Hawkwind. I still haven't quite forgiven them for ruining "Hassan I Sabbah" and "Assault & Battery.")
Skyclad again, this time with their album "Prince of the Poverty Line" from 1994. It should be noted that around this time Martin Walkyier actually was homeless for a while, well, when he wasn't in a completely dilapidated flat in Newcastle with beans on toast for breakfast, lunch, and dinner and 19 pence in the electricity meter because his former record label stuffed him out of his royalties when he was with Sabbat. It's a thumpingly good album as well and every track is about homelessness and so forth. Apart from "Sins of Emission" which is about wanking.
11. The Poetry of Charles Baudelaire
Many a goth metal band, especially soprano-and-gravel affairs with women in big flappy dresses and corsets embedded therein, smacks of having read this gentleman's oeuvres too much, but there's a few cases where bands have covered his poems outright. The little known black metal band Hasserbem covered "Chant de l'automne," and also Celtic Frost covered "Tristesses de la lune." It's actually quite good. Get a nice buzzsaw guitar tone and put it in a minor key and pop open "les Fleurs du mal" for some instant album fodder. Although not a Baudelaire cover itself, but illustrative of how certain bands have been reading too much of him, is Cradle of Filth and "Summer Dying Fast" which is something about which Baudelaire wrote from time to time.
I think eleven is an auspicious number to stop on. But I hope now you, oh aspiring headbanger band member, will now be able to do something other than crowing about being raped by Satan for once. Even though a song about being raped by Satan would be awesome in the right circumstances - get some thrashing and double-bass in there and a solo with squelchy sound effects and screaming in agony over the top... but I digress.