On one hand, I feel that God of War has exhausted Greek mythology pretty thouroughly by this point. Between the deaths of Zeus, Poseidan, Hades, Hera, Helios, Hermes, Athena, Ares, Prometheus, Perseus, and Hercules, thee are hardly any big names left to kill off.
That said, Sony Santa Monica did a great job with the three numbered games, destroying my expecctations of what the PS2 and PS3 were capable of. I don't doubt that they coujld do the same with God of War 4 on the PS4. They just need to reboot the series, pick a new protagonist, and/or find a new pantheon to slaughter.
As for what mythology to use, I suspect that Norse mythology could be a bit difficult. A lot of the contemporary fantasy cliches are derived from Norse mythology, so players could spend a lot of the game finding ideas unoriginal or comparing moments to other games and media. Between Lord of the Rings, Warcraft, Marvel's Thor, Dungeons & Dragons, and so on, you can't get away with casually dropping names like "Loki" without people having pre-conceived notions. I also don't think Abrahamic/Biblical mythology would be a good idea. Unless you're making a JRPG, killing the Christian God does not get you many friends. Especially since Bayonetta and other action games already have angelic and demonic foes.
------
Egyptian mythology would be cool, but something I'd rather see the series go with Mesoamerican/Mexican/Aztec mythology. I apologize in advance for my avoiding the actual names of these guys, but I can't spell any of them.
The good thing about Aztec mythology in terms of God of War is that there was no obvious good or bad side. The main dynamic was between the Serpent God of the sun, wind, knowledge, and art, and the Mirror God of night, magic, earth, and chaos. Both sides were equally helpful and harmful to humanity. Another one was the God of Fertility, who created the corn that everybody needed to survive, but also demanded human sacrifices so that he could make a suit out of their skins. The God of War's symbol was the adorable Hummingbird. The Trickster who started wars for lulz also had the closest relationship with his worshippers.
The gods in Aztec mythology had a long history, with there being four earlier incarnations of world. Each had their own unique quirks, like being inhabited by giants, having a black sun, etc. Each version ended violently, via being eaten by a giant jaguar, flooded by a Noah's Ark scale hurricane, fire raining down from the heavens... you know, the cool stuff. Humanity as we know it was created using the crushed bones of prior races mixed with the blood of the Serpent God mentioned above. The American continent was formed from the strangled corpse of an otherworldly abomination from the deep.
And of course, the sacrifice. The Aztec were famous for sacrificing people in a bloody fashion. There was a reason for this though. The Gods were powerful, but other than the absent creator, were far from omnipotent. The Mirror God actually lost his foot when fighting the great beast that formed the Americas. In order to have a sun strong enough to grow crops in AND move it around the world, they needed extra energy from humans. Sacrificed humans. Being sacrificed was in many cases (though not always) an honor, since you were helping everybody else in the world.
This is where our new protagonist comes in. Let's call him "Tim" for my convenience. Tim is a proud Aztec warrior who fights for his people and ends up getting involved in some mythical trouble, much like Kratos. The big difference is that neither he nor the player know what to make of all of the backstabbing and plots going on. Maybe Tim chooses to serve a god like Kratos did and likewise also swears vengeance after being wronged. But then Tim realizes that God tricked him for a good reason, or as an acceptable exchange for something else. Maybe Tim goes from faction to faction, trying to make sense of a violent world where the only alternative to human suffering is the end of existence. Like the first God of War was similar in some ways to a Greek tragedy, this game could be one where the protagonist is incapable of changing the world to their liking, even as they fundamentally change it. The tragedy comes not from the futility of his actions, but rather how any course he takes, regardless of faction and morality, only seems to make things worse.
THAT would be the perfect mythology for a new God of War.
Love and tolerate.
