Charlemagne's Vault on Mars would be cool. Edit: Also, if you read the Grimoire for each element they state that "the universe is defined by fundamental forces". As it turns out there are four fundamental forces of nature: [quote]There are four conventionally accepted fundamental interactions—gravitational, electromagnetic, strong nuclear, and weak nuclear[/quote] Gravitational is Void, strong nuclear is Solar, and electromagnetic is Arc. If Bungie is actually following this, there should be a fourth element. Weak nuclear force is actually pretty similar to something like poison or the effect Thorn has. Not saying every subclass needs an insane DoT, but this would make for an interesting theory for another subclass.