FredbearsFright is the theory that Fazbear's Fright and Fredbear's Family Diner are the same location, reused and reopened many years later

Evidence

• In FNaF: Secret of the Mimic. The R&D department of Murray's Costume Manor has blueprints that match Fazbear's Fright, implying it could be an older location than originally thought.
• As Freddy Fazbears Pizza opens in 1983, it's possible that these blueprints are of Fredbears Family Diner as it is confirmed to be open at the time.
• In the Tales From the Pizzaplex story Pressure, a recreation of Fazbear's Fright is built in the Pizzaplex, showing some sort of connection between the two buildings.
• Assuming Fazburger, it would then make sense that Fazbear's Fright could be another location.
• In Return to the Pit, when Oswald enters an arcade to start a party similar to the Happiest Day, he finds himself in an 8-bit version Freddy Fazbears Pizza.
• Since Oswald had entered the arcade from Freddy Fazbears Pizza, then assuming HDPostFNaF3, the fact that the Happiest Day in FNaF 3 appears to take place in a reversed and 8-bit version of Fredbears Family Diner, could mean that Fazbear's Fright and Fredbears Family Diner are the same location.
• The Happiest Day minigames also seem related to the way you find them, like the Shadow Bonnie minigame being entered by clicking on a Shadow Bonnie figure. The STAGE01 minigame which shows Fredbears Family Diner is found by interacting with the walls of Fazbear's Fright.
• Assuming BVReceiver, it would make sense for the Bite Victims soul to be put to rest at the same location he died.
• William somehow knows the way to the office, which would make sense if this used to be a restaurant he created.
• In Security Breach, an arcade machine called Fredbear's Nom Nom Bits. It has an image of Fredbear and Springbonnie as spirits coming out of a burning building. Due to the theme of the arcade machine, it's likely this building is suppose to be Fredbear's which implies it has been set on fire before. Since it isn't implied anywhere that Fredbear's was set on fire pre-1983, this could imply it's the Fazbear's Fright building as that game had the building end in a fire.