The fate of Kratos's brother was originally teased as a concept for a future God of War game in a bonus scene of the first God of War game. Although he was briefly referenced in God of War III, this is his brother's first appearance.
A lot of what appears in the game was originally worked on in previous God of War games. Atlantis, an entire environment in the game, was originally going to appear in God of War II (2007).
The scene with Kratos and his mother where Kratos looked for answers to his past and his mother turned into a beast he had to kill was originally a bonus scene in the original God of War game. The scene in the game is almost identical to the bonus movie, although Kratos does not learn the identity of his father in it at the end.