(the rest of this is theory. don't take me or this picture too seriously)
So, is Marionette back? Based on what Scott has given us, it's likely that he is here as one of the characters.
"He will come back."
"He always does."
Who is "he"? Just looking at it, he's talking about G.B. Look at his eyes, however, and they look surprisingly a lot like Marionette's when you get jumpscared. There is a theory that Marionette is behind the entire story line. While we only see him once in the history, this could be true. In the first game we have a lot of support for this. We can sometimes see pictures of a crying child down a hallway, which looks the same as the child that was killed in the "cake" mini-game in the second. This child could be the one possessing him. Many years later, most likely twenty, is the first game. The animatronics from the old place were scrapped, so why is he there? There are two ideas I have: He possesses the new place, or he stayed there and is watching Mike. He could be controlling the new animatronics and sending them out to kill you, maybe thinking that it will set him free so his spirit can move on. Mike escapes, leaving us to now jump thirty years into the future. Marionette goes back to his old way of killing you and goes after you himself. Due to being hidden for years, he has broken and can't alone. He puts himself in another animatronic, but how can I confirm this? When you find G.B. you can see he has a similar shape to Mar.
Phone guy said that somebody used spare animatronic parts that were in the back. "A yellow one." A yellow what, exactly? He never said, but this could possibly be a golden rabbit. It was thrown back and hidden away, never to be used, and we can see that G.B. is broken. So if it isn't Marionette, then it makes sense for it to be G.B. Maybe the animatronics knew he was back there and that he was going to come back.
Is he the leader of this?
Will the full history be revealed?
Will we find out who Phone Guy is or was? Was he the one who killed the children?