Hmm, great cosmic threat to hero's people, must get reluctant hero to save all humanity as we know it from fearsome monster (or race of monsters)?
Nope, never been done before. Cough * Conan, Beowulf, LOTR, mythical archetypes * Cough. Excuse me. Highly recommend some Joseph Campbell, and some Robert Heinlein to your reading list. You may also enjoy,
Bill, the Galactic Hero by Harry Harrison (I believe he was originally goaded on by Robert Sheckley, whose own
Zirn Left Unguarded... is a classic of the genre). Yeah, never been done before. Totally original.
//Actually, it's quite an old war-horse.