Right from the very first teaser of Springtrap, a fair amount of people correctly guessed that he's the Purple Guy's ghost.
Yes, there's a Golden Bonnie in this game. As far as we can tell, it's who Springtrap was before he was used to trap the murderer and started rotting away.
Smosh Games predicted in their Honest Game Trailer of the third game that there was going to be a fourth game, though this was obvious considering the hype the series had at the time of release.
Similarly, after the third game, Scott released a picture of Freddy's hat sitting under a spotlight on stage; while others thought it symbolized Freddy and friends leaving the stage and that the third game was indeed the last, some fans believed that there would be more to come in the series. The latter group was proven right after the hat disappeared from the picture and Nightmare Freddy turned up, confirming another game was in store.
Many fans theorized that the game would star the Bite victim (although they weren't completely right) and/or would be a prequel.
After the third game, there were two popular theories about Golden Freddy: 1) that he's Fredbear, and 2) that he's the second springlock suit. This game confirms both.
When Nightmare BB was confirmed, it was expected that a Nightmare Puppet would follow. Soon after, a darkened teaser revealed it lurking on the bed when brightened.
Some people correctly guessed that the "lines" teaser was hinting the title "Sister Location" and it was a new game in the Five Nights at Freddy's series.
Before her name was confirmed, "Ballora" was suspected to be the name of the ballerina animatronic in the trailer thanks to copyright leaks. Then a hidden message on Scott's website on June 17th showed abbreviated names for the new animatronics seen at that point, which shows abbreviated names like Baby being listed as (ANM01BBY). In this case, you could also see ANM02BAL in the source code, confirming the ballerina animatronic's name.
In addition to the leaked names, fans were correct on the characters that were teased in the images, going from Baby, Ennard, Bidybab, and Minireena respectively.
MatPat predicted, in his video attempting to predict the game's story, that the new animatronics were designed by the Purple Guy. In Five Nights at Freddy's: The Silver Eyes, the Purple Guy's name is William Afton, and the game implies that Mr. Afton, who made the circus animatronics, is Purple Guy. In one form or another.
In a livestream, MatPat posited the existence of multiple Purple Guys. Scott intervened, saying they were getting too close to the truth.
Just as fans had guessed, Ballora is blind, and her strategy involves noise.
Many people, including MatPat, theorized in some of the early teasers that Vanny and Vanessa are the same person, controlled by Glitchtrap. When the game was released, the theory was confirmed through further implications and two separate endings that leave no room for ambiguity. Even the Dummied Out Vanny meter seems to indicate it as when filled, Vanessa's model is switched for Vanny's. Despite some people still thinking they were separate, the official Five Nights at Freddy's Character Encyclopedia treats Vanny and Vanessa as the same, further clearing up any confusion.
Once teasers focusing on Fredbear's Family Diner were released, it was a common theory that the Pizzaplex was built on top of it, the Final Boss battle would take place there, and that it'd be a new form of William Afton. Well, it wasn't built on top of Fredbear's, but besides that everything is correct.
When the trailer for the Ruin DLC came out, a lot of YouTube comments correctly guessed that the person Cassie was talking to was just a robot pretending to be Gregory. Doubly so for people who guessed it was the Mimic.
As soon as the synopsis for Step Closer was published, a large number of fans immediately guessed that the Susie alluded to in the synopsis for the third story as the same girl possessing Chica in the game and novel continuity. Well, kinda. She is clearly not the same Susie, as she died in present times, not in the 80's, but besides that, she is clearly supposed to represent her and confirm the "Chica is Susie" theory.
Similarly, upon the publication of the synopsis of Bunny Call, fans almost instantly jumped on the idea that the Man in Room 1280 was either William Afton or his son Michael, either one having survived the fire in Freddy Fazbear's Pizzeria Simulator. Come the release of the book, and it's heavily implied that the former is the case; the sixth volume would essentially confirm this in its Epilogue.
