They should stick with Greek Mythology and Kratos can't be replaced. He's far too popular and beloved to be dropped, seriously all of Sony's other characters combined aren't close to matching Kratos' popularity.
And here's a look at the 5 most popular characters from exclusives on PlayStation and Xbox. Note, Kratos is MORE popular than Master Chief. Why would Sony drop such an iconic Kratos when in the real world only the insignificant minority want a new character.
I think God of War IV should be a direct sequel to God of War III and have the Primordials (http://en.wikipedia.org/wiki/Greek_primordial_deities) as the main antagonists now that Kratos had basically freed them by killing the Gods responsible for keeping their attributes under control, a.k.a. Zeus kept the world in order but now that Kratos killed him Chaos is no free to rise again (which we see at the end of GoW3).
Nothing else in Greek Mythology (or any other Mythology) is as powerful or interesting as them and that could make for brilliantly epic encounters (especially with PS4's amazing visual & processing capabilities) and Chaos itself being the new big bad guy/thing is just epic. Instead of Kratos seeking to take them down for revenge, now that the power of hope set him free and he's found the strength to forgive himself he wouldn't be angry anymore, he could have a new reformed attitude and more humanity so maybe he'd actually want to defeat Chaos and restore the Earth back to a habitable state where humans can thrive again. Perhaps Kratos could even fall in love again and have a son who you play as in the future.
Greek Mythology is so vast & intriguing therefore I want them to stick with it, if they don't I'd prefer if they chose Egyptian Mythology next. And Kratos can't be ditched, that's like Nintendo or Microsoft dropping Mario and Master Chief, crazy idea.
