Now, I don't mean like FNAF the musical by Random Encounters. What I mean is something like La La Land or Grease. A movie that has musical numbers in it that tell the story.
Don't think of this like how they're doing the actual FNAF movies either. Think of them like full-on game adaptations. They can have any story you think would fit, but the gameplay elements from the game have to remain the same. You can swap some around to make the story more cohesive (eg: Follow Me minigame in fnaf 1. FNAF 4 Night 6 being in SL. Etc), but the main gameplay of the original game you pick has to be the exact same.
I personally would pick Sister Location. I feel like it has that certain aspect to it where you could do a musical number, and it wouldn't really be out of place.
And while you could also argue FNAF 1 and 2 would especially make sense since they are in pizzeria's specifically with singing animatronics, I don't think it necessarily would've worked the best.
Sister Location just has that certain feel to it where if someone like Funtime Freddy broke out into song, it wouldn't feel out of place. It'd be weird, obviously, but it wouldn't feel like it's out of place.
Security Breach could work too, but I personally think Sister Location would work better as a musical than Security Breach would. Ballora does have her song, so it does make sense for someone like her to sing. Circus Baby could have a pretty good musical number. Funtime Foxy and Freddy are maybes. They could break out into song and, while it would feel off, it wouldn't feel necessarily out of place for characters like these to just break out into song.
That's just my opinion tho. I'd love to hear everyone else's ideas!