There were a ton of theories and supposed leaks about what was going to be in the movie. Here are a few.
1. Snoke was really Darth Plagueis, Jar Jar Binks, Leia Organa, or Donald Trump. Actually, the movie doesn't explain who Snoke is, although I have heard that the comics give some of his back story.
2. Luke tells Rey that she is his father. This would have been an ironic twist, but the supposed leak was that she is the reincarnation of Anakin Skywalker. The story went that "The Chosen One" reincarnates and Rey was the reincarnation.
I was hoping desperately that this would not be the case, because it would have been one of the corniest lines in all of Star Wars.
There were so many incorrect leaks from supposedly reliable sources, that I suspect that Lucasfilm might have been disseminating false information to throw off all the leakers. This one sounds like an April Fool's joke.
Some of the later trailers seemed to have been cut in a way to deliberately mislead about certain plot elements. However, many observant fans quickly figured this out.
3. Luke tells Rey that there is so much more to the Force than the Jedi way. This actually was in the first trailer, but I don't recall hearing it into the movie, just as The Force Awakens and Rogue One have scenes in their trailers that didn't make it into the movies. Often trailers are released before the final editing of the movies are completed.
This lead to the popular theory that Luke was a "Gray Jedi", which is to say both light and dark. The concept of a "Gray" has been explored in other Star Wars media.
4. Kylo Ren and the Knights of Ren visit Acho-To and do battle with Luke. He quickly dispatches the Knights of Ren with a new "push" force power, and then does battle with Kylo Ren.
There are people who claimed that they saw this scene being filmed. It is possible that the scene was filmed, but not used.
In the original films, there were several scenes that were filmed that were also not used.
The first cut of the movie was a whopping three hours long and had to be trimmed down to 2.5 hours, which still makes it the longest Star Wars movie.
5. Either Anakin Skywalker, or Darth Vader, or both, appear as a force ghost.
There were sources claiming that the actor Hayden Christensen was seen on the set of The Last Jedi. Likewise, Hayden Christensen came out of hiding to make public appearances where he claimed that Star Wars was still part of his life.
This could still happen in the next film.
6. As part of a test, Rey has to go underwater to fight a sea monster.
Again, people claimed that they saw this scene being filmed.
7. I'm not convinced that we know the truth yet about Rey's parents. Kylo Ren could have been lying, but on the other hand I thought Darth Vader was lying too.
In an interview, Adam Driver actually referred to Rey as a princess in hiding pretending to be a nobody. This confused a great many people, but some thought that this might be deliberate misinformation, and others thought that Adam Driver was speaking figuratively.
There was nothing in this film to indicate that this might be true. Still could happen, but it doesn't seem likely based upon what we have seen so far.
Daisy Ridley claimed that Rey's parents were obvious to her after filming The Force Awakens and thought that everybody would figure it out, but that doesn't appear to be so obvious from what we have seen in the movies.
The shuttle that leaves a young Rey behind on Jakku looks so much like Luke's shuttle in some of the comics. There were many things that made people think that Luke was Rey's father, but in the movie he doesn't seem to know who she is.