For the plot to really make sense there needs to be a fourth game. A prequal to FNaF 2 could close some very large plot holes in the story.
The game could take place at the original Freddy Fazbear location, The one after Fredbears family diner and before the FNaF 2 location. This would "explain" the original first murder (that lead to puppets possible possession) that the classic animatronics were decomissioned for and explain the reopening of the 87' Freddy location (where 2 takes place).
In FNaF 2 night 1 ( I Believe ) our good pal phone guy speaks of the location before 87' saying how it was left to rot. Playing in this location could open up some decent origin information that could be beneficial to the story as a whole.
This could also go with the current odd format of the story, odd numbers being current and even numbers being prequals.
The golden bonnie and freddy suits both have no true origin, before 3s release we all thought the murderer hid in golden freddy causing the murders as well as golden freddy being possessed by a fifth child. :SPOILER: There was a golden bonnie suit in the original Fnaf 1 location (SpringTrap) Where the murderer hid inside when he released the childrens spirits when dismantling the classic animatronics at FNaf 1's location. What were the golden suits used for? or are they just gold because of age? Perhaps FNaF 4 could go into detail with this also bringing the series too its roots. Maybe even explaining who the murderer is. After all hes been kidnapping/ murdering kids for over 40 years now as well as ruining this companies name.
Could it be phone guy? sofar (as much as i dont want to admit it) the majority of the signs point to him. Someone whos been with the company since the early 70's perhaps as an intern, someone who went to fredbears family diner as a kid falling in love with foxy the pirate, someone who perhaps after years of never becoming a manager or anything higher than a security guard..after years of torment and fear from the anamatronics.... snapped under pressure murdering one kid opening up a dark side that leads too 5 more kids...who follow him up until FNaF 1. Where the animatronics souls are released and thus killing him getting revenge.
Its clearer now that i have written it out than ever before.
I think scott can make another couple hundred thousand dollars clearing up this issue with a fourth game.