So, basically there is a lot of mystery on the locked box from fnaf 4 and same with Cassidy, aka Golden Freddy.

So I decided to analize everything and publish my own theory.

So, who is Cassidy? Game doesn`t reveal that. But what if she could be a member of Afton family? What if they had the fourth child?

What if one day aftons went to Fredbear`s family diner with Cassidy, and than tragedy happened? Phone guy in fnaf 3 says that in first restraunt there were "Springlock failures". Maybe this could be connected to Golden Freddy, who accidentally killed Cassidy becuz of this failure?

This could be an answer why William was so overprotective to all other children. He didn`t let Elizabeth go near Baby, because he knew that his another child can die because of animatronic.

And what is inside the mysterious box?

These are personal belongings of Cassidy. As note says "some things are best left forgotten", can mean that William locked everything that can remind of a Cassidy.

He wanted to fogot her to not plunge into sorrow again.