Based on the secret ending to God of War, it's likely that Kratos will be facing off with Thor as the events of Ragnarok seemingly move forward. There's a ton of questions about that and about how Atreus (since he is Loki) could be The World Serpent's father, and it's not impossible that time travel returns to the franchise in some form. Regardless, Thor probably isn't the biggest threat Kratos will face.
The battle between two blades forged for each other, Kratos' Leviathan Axe and Thor's Mjolnir, may just be one battle that is put to shame when Kratos eventually faces Odin and Gungnir. While it remains to seen how Odin manifests in the next God of War, he's likely to be incredibly powerful in terms of magic and perhaps even physical strength (he's normally depicted as a old man), but there's at least one big reason he'll appear in the sequel before the presumed threequel: Zeus.
Kratos and Zeus
As leaders of their respective pantheons, Zeus is much like Odin in God of War. As such, it wouldn't be surprising if Odin were the main antagonist which seems pretty apparent. In many ways, the overarching narrative has them playing the same role. In the original God of War, Zeus does appear to Kratos, giving him the ability to wield lightning bolts and playing a small role in the story. Zeus is mostly an idea, here, though, with the character playing the role of an observer, guiding the story but not directly involved (at least to Kratos' knowledge thus far). Kratos didn't even know he was Zeus' son at this point.
Seeing the original God of War as a potential parallel to the new one, it's clear that Odin's involvement in the story is the same. Mimir talks about Odin a lot, and the family connections are obvious. However, Odin never manifests in the flesh (as seen so far, any way), with his ravens instead watching Kratos and Atreus at every turn. It would seem that Odin is observing Kratos, simply without the conviction and manipulation seen by Zeus. That doesn't mean he isn't, though, and it'll be interesting to see how much the Allfather has been involved come the sequel.
Kratos and Odin
Whether the game retells the events of Ragnarok in God of War, somehow diverts the end of the war through time travel (yet again), or is simply more build-up to Ragnarok, it's unlikely that Odin isn't involved. It's assumed that Thor wants revenge against Kratos and could be seeking him out himself, but it could also be that Odin sent him. Beyond that, it was the second God of War that revealed Zeus' betrayal and set him up as the true antagonist of the original trilogy, which lines up nicely with the upcoming God of War Ragnarok sequel.
Of course, Santa Monica Studio could throw fans a curve ball. The mural at the end of God of War shows Atreus seemingly killing Kratos, but it could be that the runes indicate Atreus will kill Odin. As such, it's not impossible that Atreus becomes an antagonistic force, but it's clear that Kratos' life is also on the line. If not for that, then the Cycle of Patricide as seen in the franchise. Fans may have just to wait and see where the story goes next, but for many, that wait for more information may just be killer itself.
A new God of War is in development for the PS5 and is slated to release in 2021.
