So basically Mike will check on Afton locked in the safe room after getting some like nightmares or anxieties of William making his promise to come back and after that, he'll forget to seal the saferoom up again letting Afton escape then he'll kidnap Abby but luring her away or something.

Then he'll take Abby to the sister location, sending Mike a letter or a calling card about where he is, Mike will need to do some research either getting from online or from the primary source herself Vannessa who is presumably going to still be in the hospital, Mike will find the location and will do anything William says to try and get Abby while the Funtimes try to get quirky at night and capture him, and the Puppet will help Mike get his sister back.

It'll basically play out like sister location but William is an added active threat and is the motivation for Mike being there and basically staying there for multiple nights.

Anyways, after Mike completes what William says, William reveals that he already stuffed Abby into circus Baby with no memory who she is and shows Mike a video of how he did it breaking him, but all Baby wants is to get out, Springtrap will capture the Puppet in Lefty, then Baby will lead Mike to the scooping room, deceiving him with all the other Funtimes, kill Mike but removing his innards, use him as a skin suit and escape leaving Springtrap stuck in the sister location who'll inevitably get out and then Ennard will leave Mike's body after it has roted to be purple but Mike will somehow come back to life, Baby gets kicked out of Ennard, Ennard goes to the FNAF 1 location and Vannessa will be wherever she is, still presumably in the hospital

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This is basically a simplified version of what I think will happen in the second movie, since FNAF 2 is less likely to happen since FNAF 2 is a prequel and the FNAF 1 location, in the movie is where the children were murdered since that's why it was closed down meaning the FNAF 2 is not a good continuation because of the withereds so Sister location is our best bet setting up for either for FNAF 3 or FANF pizzeria simulator

Springtrap will most likely be treated as how he's treated on the books so he's a more active threat and be more developed on and what he's really done, maybe we'll get a glimpse at the Mimic or GlitchTrap

But don't take my word for it, I'm not involved with the movie and this is just something I put together in a short amount of time with the information I have from the movie

But hey, that's just a theory, A FILM THEORY!

Anyways, thanks for reading this, it probably got a bit wordy