I don't have the time to hoast an entire arpg so I i figured id just get the ideas out there instead just in case someone is inspired enough to take up the concept!
I haven't personally played much fnaf but I do love watching videos of it and stuff about the lore <3

setting - 
My main thoughts on how this would be set up from a background point is that this is either 1. a Disneyland-sized amusement park. or 2. People would design their own locations with an overarching theme similar to a lot of warrior cats arpg's I've seen. 
1 . For the amusement park idea, the background would be more controlled by whoever runs it? but people could probably still make up their own rides or work together to build a restaurant or two. I know Disney has dozens of restaurants, rides, and even stores. so basing it on that would give people tons of room to have as many animatronics as they want. humans and ghosts of course! 
2. For letting people design their own locations, it would be kind of free forum. you can make whatever type of attraction you wana. then level it up by doing various tasks. the more you level up the more you can add to it. like you start out with a tiny place and one animatronic character slot. but as you do tasks, prompts, and generally build up your own lore you earn currency and more character slots. 

I can see map making would be a fun part of this, you'd get to design your own location or building maps and earn currency and xp for drawing it yourself. 

characters - 
characters would probably be in a few different groups. you can design them any way you want! but maybe have to roll one or two things for them? stuff like job type. or maybe humans have to try and apply for a job xD
groups would be what species they are. animatronics, humans, ghosts or undead. could even do like plushies or worker bots
could have a jobs section where characters can take a job and fill out the current given prompt to get character xp and improve stats. 
stats would be fun, not necessarily just normal dnd stats but random stuff like aggression level, fear level etc.

Thank yall for taking the time to read this mini rant asghas
feel free to converse and share ideas! Consider this brainstorming <3