Obviously, as you mentioned, there's the reference to Kratos's brother in the special features in God Of War I. Can't really be considered to be a confirmed canonical story element, as the special features also have the bit about Cronos living another 1000 years, which he obviously didn't...
In the ending (post-credits) cutscene of God Of War: Chains Of Olympus, Athena makes a comment: "He'll live...they must." It's never been explained who "they" refers to.
In Hades in God Of War 3, you find a note from someone who is apparently (not confirmed, but given that all the other notes you find are from people who have interacted with Kratos in one way or another, I don't know who else it could be) Kratos's mother, stating how she tried her best to raise him, but listing his evil deeds...one of which is killing his brother. This is clearly not a reference to Ares (who I guess was technically his half-brother), as the note implies it happens before Kratos killed his wife and child.
Just before the last battle against Zeus, when Gaia shows up, Zeus taunts her with a comment about "her pawn failing" or some such, also stating, "Perhaps you should have chosen the other." Who could "the other" be referring to?
So my theory is that God Of War IV will focus on the story of the "grown-up-in-the-underworld" brother of Kratos rising to prominence and/or seeking revenge now that the world is in chaos. After all, it's been said by the devs essentially that "this is NOT the last God Of War game, but it IS the end of Kratos's story arc."
Makes sense to me, anyway...what do you think?