A VR game was later made, that was a remake of these, but some suspecious circuit boards were sent to be scanned into it, creating a paranormal, digital entity known as Glitchtrap. It appears as a man in Spring Bonnie (William's suit), acts exactly like William, knows stuff only William knows and even claims to be back, with William's catchphrase of sorts being "I always come back". After a bit of shenanigans, it manages to possess a beta tester and makes her his minion. We call her Vanny.
Vanessa, most likely Vanny, then got a desk job at Fazbear Entertainment. Fazbear Funtime Service, a program where animatronics were mass produced and sent for parties, etc. got hijacked by Glitchtrap who made them hostile. He made Vanessa infiltrate the service as a fake FazEnt representative, supposedly fix his infection, but actually just cut off anyone's access to the system other than him, and send a data packet with him to the current Pizzeria — Freddy Fazbear's Mega Pizzaplex.
In the current book series, most likely canon, there's this AI called Mimic1, which copies stuff. Many think that's actually what Glitchtrap is, just AI copying William. It's an ongoing debate. An Endoskeleton with it installed was sent to the last simulated Freddy's Henry built to burn, where it got out of control, started killing people and in the ongoing story is hunting down a group of teens. As the Pizzaplex was built on top of the simulated Freddy's, the Mimic got control over it too, but was quickly taken down. There's also this boy Gregory who ended up possessed by Glitchtrap and hacking the Pizzaplex for reasons unknown.