Man, I've been knee-deep in that corner of the FNAF fandom for a while now. The most common thread I see is the 'Reluctant Possession' arc. William, forever trapped as Glitchtrap in the digital hellscape, finds a way to partially manipulate or fully inhabit the mind of someone—often a game developer or a poor soul who played the VR title. The tension isn't just horror; it's this psychological push-and-pull where William's cruel, calculating consciousness shares headspace with another person's fading sense of self. It's less about jumpscares and more about the slow erosion of sanity.
Another huge one is the 'Digital Hell Reconciliation' between William and his son, Michael Afton. Writers love to throw Glitchtrap-William into a simulated version of the Pizzeria or his old family home, forcing confrontations with a ghostly or also-trapped Michael. The angst potential is off the charts—unresolved paternal hatred, guilt over the Bite, Michael's own rotting body versus William's digital corruption. It often bends towards either a tragic, mutually destructive ending or a bizarre, twisted form of redemption through shared eternal torment.
Then you've got the 'Merger' storyline, which gets really metaphysical. Instead of two distinct entities, William and the Glitchtrap program integrate so completely they become a new, third being. This opens up wild AUs where this composite creature tries to rebuild the Afton legacy from inside the net, creating digital animatronics or luring others into its server-prison. It's a favorite for writers who want to explore identity dissolution beyond a simple villain possession narrative.