Allow me to explain; Needless to say, Spoilers Ahead for those who have not played through the first three games.
Kratos has been the poster child anti-hero since the franchise's inception in 2005. By no means sympathetic, by no means merciful, just brutal and to the point. He does not consider himself a hero, nor are his actions reflective of someone we should care about; yet for three installments we controlled his quest for vengeance.
My sympathy and compassion for Kratos diminished as the franchise continued.
"I should have pressed 'O' faster"
In the first installment, I pitied and rooted for Kratos. A man who served the gods, setup by his own lust for power, ultimately destroying the family he loved without even knowing it. It was a sad fate, and his quest to kill the very deity who led to his state of torment was ambitious. Through the entire game, myself and the other Gods, were rooting for Kratos to come out on top. Sure he was not very sympathetic to that boat captain at the beginning, and his methods were fierce, but when everything is taken from you it made sense to rip the head off of a medusa and use it as a weapon. Even after defeating Ares, you felt a twinge of anger towards to Gods for using him as their puppet. Despite leading him astray their reward for Kratos of giving him the seat of Ares seemed like a solid resolution.
Then the second game threw a wrench in that pity. All that power and no real purpose went to Kratos's head, and despite warnings from Athena, his arrogance cost him his place in Olympus. You could argue that he was never truly over the trick of not wiping his memory of past events as promised in the first game or the bitterness of seeing senseless wars over the ages. Regardless, he cast anything he learned from the first game aside and went on a rampage. Even at the end of his mistake, Zeus offered him one final chance to return to his status and play by the rules. He ignored this, and it cost him all he created. Sure, Zeus is ruling with an iron fist and gave him a "my way or the highway" ultimatum to a person who cannot be dictated.
"I can't just use one shoe, Hermes, that's silly"
The only motivation Kratos had for killing Zeus at that point was the destruction of Sparta and his personal view that Olympus was useless. His family was already gone, his buddy Athena was having trouble vying for him, and all he had was a hatred of Zeus for crossing his path. The source of motivation flipped. Kratos, this once fearful god killer, was now on a killing spree to end Olympus. Not for anyone else, not for humanity as a whole, but for his own personal vengeance.
"Whatever", I said to myself, "Zeus crossed me. He wiped out my army. You do not do that to the first mortal to kill a god. Gaia just showed me how crazy he was to his own children. Let us go show him why he made a mistake." That became harder to justify when even at the end, with the potential to go back in time to save his family, he chooses to go back and beat the crap out of Zeus.
Then God of War 3 entered the fray and skewed my vision of Kratos further. Lesser of two evils kept me going, while he was not good, he was not nearly as absolute and evil as Zeus seemed. Yet, somewhere between ripping off Helios's head and slowly walking toward a limp Hermes whose leg I just amputated, I became jaded to Kratos entirely. The attempt to humanize him again with his care for Pandora and psyche trip with his family at the end did little to help me see this character's justification anymore. Knocking out Gods and flooding the world, blotting out the sun, bringing disease to the world; he showed no sign of remorse or sadness. Maybe Athena's promise of world without Gods helped him rest easier, but still...not a twinge of regret for the life lost.
You could argue all day about whether Kratos was the hero, there are dozens of blogs about it, which made the franchise that much more memorable. Everyone was morally gray, no true sides, just people getting power and using it as they wished.
After the Norse Gods, we take on the Avengers
God of War 4 is all but inevitable, and leaked art has confirmed a possibility of Norse Gods like Thor and Odin. Just putting Kratos toward these gods as a massacre tour 2016 would be interesting, but hard to form a motivation behind. The character has all the potential to carry it forward with his bitterness from constant betrayal from self seeking Gods and Titans to go on a deity killing spree. Perhaps he now seeks the destruction of all gods from the world? This would require delving into Egyptian, Roman, etc?
Instead, I would love a game in which Kratos has risen to power and must be stopped. All the ingredients are there from the previous games; the short fused temper, the lack of empathy, the little remorse at life that does not pertain to his own agenda. He could easily rise up as the villain we would love to take on and with a new protagonist it could be potentially great. Maybe one whose family was killed in a past Spartan conquest or destroyed by his actions from the third game. I see such potential with this premise and where the franchise could go.
All of the best villains started out good; Sephiroth, Darth Vader, and even Wheatly from Portal 2. There is something special about seeing the good of a character and the stark extremes they go to after traumatic events. It makes them memorable, it makes them justified. It makes them evil, but with good reason. Seeing Kratos's journey across three games, seeing the events he has had to push through, and seeing him come out from that unscathed would be unimaginable. I could see a much more cynical, destructive force emerging from the events of the trilogy.
Regardless of how it turns out I know I will be excited to see if Kratos even makes a return, though I really would love to go toe to toe with the god killer himself.