God of War's Kratos alwaysd had an aura of invincibility, and during the God of War Retrospective panel at PlayStation Experience one of the attendees asked if Kratos can be defeated.

Game Director Cory Barlog had a quite interesting answer to give.

Is Kratos Beatable... I mean, he beat himself in the first game... multiple versions of himself? Maybe not in this mythological pantheon.

Of course this can be interpreted in several ways. Kratos could simply be invincible, or Barlog has considered or is considering a different mythological setting for the series. I'm sure some could even think that this is a hint that the next God of War could be set in a completely different mythology. It's a quite open, but intriguing answer.

Personally (and this is just personal speculation, so don't take it as gold) I feel that if Sony Santa Monica ever wanted to create a new God of War with a new protagonist, he'd probably have to defeat Kratos in some form.