Thor breaks the Marvel universe. Just look at the lineup that we have going into it: Fantastic Four, The Hulk, X-Men, Spider-man, Daredevil, Iron Man, Dr. Pym (Ant Man/Wasp), Hawkeye, Captain America, and many many more.
Every single hero on that list is either one of two things: A scientist who got caught up in an experiment gone wrong, or a regular (or irregular) guy who came across a strange set of circumstances and now chooses to deal with it by being a hero.
And then there is Thor. While he does technically fall into the second of the categories that I listed, the powers that he attains make him LITERALLY a god. It changes the entire mythology of the Marvel universe(s), in a way that, in my opinion, is not for the better.