Yay, a fellow FNaF fan! *high fives*
My little sister and I were having one of our long, drawn out video game discussions about this particular teaser (because she's an amazing kid and we'll both take over the world with our combined knowledge of such things... probably.)
Anyway, I immediately noticed that the window looked like one you'd find in a house, as opposed to a pizza or theme park establishment.
"Plushtrap's" resemblance to a kid opens up a variety of horrifying possibilities. Originally, it's been theorized that one would play from the perspective of the Bite of 87 victim. However, the presence of Springtrap seems to negate that: He didn't surface until Phone Dude fished him out of that room in FNaF 3. This teaser gives us a sense of the games setting, particularly its placement in the timeline.
I propose that after the fire in FNaF 3, someone bought Springtrap's remains in the subsequent auction... and gave it to their kid.
What if the kid put on the mask and a Majora's Mask scenario happened. Straight up possession, yo. And, we get to see what the kid sees.
Nightmares. Of course, this is just an idea, and I could be
WAAAAY off.
(Also, if you play with the threshhold of the picture, you can see the kid is in a hall with a bunch of doors.
)