BBV

Summary
This theory suggests that CC possesses BB after death.

Proof
This theory raises doubts that CC would have inhabited an animatronic that the game clearly showed him to be terrified off-that being Fredbear and Shadow Freddy (especially since the latter always appears in parts and service, where CC was deeply distressed to be)

With Spring Bonnie for all intents and purposes uninhabited, the toys non-existant yet, and assuming that the core four and GF only are vessels for the MCI, only BB remains.

BB's behavior could match CC's fear, as a look inside the game files shows that if you show BB the Freddy mask he will not regroup at the main hall like the others but flee back to his starting position. This theory also suggests that BB planting himself in the office is not a gesture of triumph but of fear, since CC would sometimes freeze in place if he was afraid, and it's impossible to tell if BB is truly laughing or trying to express something else with his limited voice box.

In FNAF6, the Fruit Punch Clown minigame places BB in an extremely similar role to CC in the FNAF4 minigames.