Regarding Kratos being Tyr it's pretty obvious that the World Serpent is from the future, Mimir says that their's a rumour that he was hit so hard by Thor during Ragnarok that he gets sent back in time. So that's a way that the time travel would work, though i do not think that Kratos will be tyr in this "time line" as it would mean that the same series of events occurred.
When it comes to my own personal theories i mainly have one, and it has to do with fate. We're told throughout the game that the giants have future seeing capabilities and i think that it's false. First of all we know that Kratos is already able to change, given what he did in the original trilogy, so his mere actions in the world will change that. Also i think that the next two games are really going to focus on "fate" being something that can be changed, such as all of the predictions about Ragnarok. So when it comes to Ragnarok the single thing that would have caused it is Odin's reaction to the predictions, and i foresee him as being blamed for the coming of the prophecy. This all makes it that the giants predictions aren't accurate, but that there mere existence paved the future.
And god your signature gives me a headache