Game would take place in the FNAF 1 location long after it is abandoned. In this universe, William Afton was the Nightguard in FNAF 1 and he hid in the springbonnie suit in the backroom to try and avoid the animatronics and their angry spirits but ended up getting springlocked. The other children wanted to move on but Cassidy wanted to stay and torture William and she wanted all of her "friends" to stay with her, so she used the Phone Guys body to attack Charlie and damage her beyond recognition and then took control over the other spirits.

In the game you would play as a kid who is tricked by a bunch of bullies who he thinks are his friends into going into the old building, where they lock him inside. The game would be just one night, with each "new night" instead being a new hour. Cassidy wants to kill the kid so she could have a new friend because she's starting to get bored and lonely.

Springtrap would also be an enemy in the game, i think. He'd walk into the office and get really close to the player and you'd have to stand still until he leaves. He'd be basically unrecognizable since i think cassidy would take genuine delight in mangling him.

In the end the bullies get nervous that if the police find you and you tell them what they did, they'd get arrested, so they set the building on fire and try to set it up to blame you. You get 2 choices, either burn to death or get killed by cassidy. burning to death leads to the normal ending and getting killed by cassidy leads to the true ending, where the kid becomes Shadow Freddy (literally Cassidy's shadow).