Since it doesn't look like anyone is actually going to explain what this would involve, here's a very simple pitch for a minor FNAF thing that could be added to Halloween:

Event: Are You Ready For Freddy?

Freddy is now on the loose!

This occurs during the same pre-event stage of the turn as the initial Chicken message. Some time within the next few turns after this, a Freddy item will silently appear in someone's inventory at random (like the Chicken appearing, but with no event text - probably best to only make this trigger during turns where people have other effects that would be mentioned at this time to disguise it). Note that Freddy won't always appear in the inventory of the player taking the current turn - he shows up for one player at random.

Item: Freddy Fazbear

Click Freddy to make him go away!

If Freddy isn't clicked within 30 seconds of appearing, Scare is cast on the player who has him in their inventory/Magic Hat and Freddy disappears for another few turns. Trying to Pawn him will make him vanish as if you had clicked him. Transmute, Flux or anything that destroys items (Fire, Lasers) will stop the event occuring until the Are You Ready event comes up again.

The idea is that he randomly Scares a player every few turns if they're not paying attention, but is otherwise just kind of a nuisance. This particular version of Scare could probably be reskinned to be Freddy's face, and the scream would ideally be the one from FNAF.

Some possible achievements:

Ready For Freddy

Kill Freddy Fazbear.

(ie via item destruction)

IT'S ME

Die while being scared by Freddy Fazbear.

I would just like to spin my own take on this plausible game mode idea. We know there have been unused game features after the files of FNAF were explored like a Toxic meter and life system. My idea first involves utilizing these into the mode.

Toxic-The more you get injured[i.e Skip A Turn from screaming, pacified from Suit Stuffing attempt] and/or use the Fazbear Mask to hide, the more likely you are to be ejected from the event and move on the regular game map after taking two turns [or less pending revival interceptions from death.] Toxic level starts at d1 to beat upon first introduction to toxicity and increases pending the exposure [hiding, one point per turn. Injury from animatronic, 2+.] The dice you roll to move must be higher or equal to toxicity lvl, if they are not then you die and are ejected back to the regular game destinations. 

Second is 'lives'. These are the lives you have pending battle against other players. After you die due to them killing you a certain amount times, you are ejected back to the regular board.

Also, as a do note, the event while seemingly a 'team event', in a free for all battle has to do with 'sharing the less money with others'. Unless of course, you don't mind working together in the event regardless of the free for all. Reward for event Winner will at a slim chance be big, but usually be 100-125 coins. If there are winners, the coin amount is split up. Event would NOT end in 5 nights as 5 turns would be boring, but will end upon meeting either one of three conditions:

-Golden Freddy is Reassembled physically 

-Work on an escape route long enough for it to be a viable means to actually escape, and disable the animatronics to prevent a pursuit

-Beat every animatronic in a duel [for Board Game Online, they all have guns.]

Now to explain each room and its functions, which these are just examples by the way and could have much more with each room:

Prize Room:

-Wake The Baby [Marionette will have random chance of either attacking the waking player or attempting to attack a different player.]

-Get Your Prize [In game currency can be used, but its cheaper to use the Achievement Stars earned from avoiding Animatronic Attacks to get fun or crucial event completion items.]

-Wind Up Doll [While players can still choose to Wake The Baby, they won't know how many turns it will ignore their attempt to wake it because of the doll.]

Parts & Services Room:

-Search for Parts [Look for Mechanical items, chance of gaining a random mechanical item, low chance of encountering a broken yet malfunctioning animatronic. Rare chance of finding Golden Freddy part or Item needed for escape.]

-Restore Power [upon successful attempt, power to building is restored and animatronic attack attempts go back to regular percentage. If someone has a way to mess with the systems [for example Sabotage or EMP Device] they can remove power from every powered device in the building allowing for escape route to work.]

Party Rooms [covering each one] can be a place to go but at risk of encountering an animatronic. However, random chance will dictate the nature of said encounter [violent or not violent] and searching can also yield a chance at getting a much needed part. You can also choose to duel an animatronic in a Party Room.

Pirate Cove:

-Challenge for the Captain [Foxy won't always be here making the duel option for Foxy that much harder.] 

-Pirate Booty! [Taking a chance on getting some pirate booty without being noticed, you attempt to loot. Loot will almost always contain either Golden Freddy piece or Escape route piece. May also trigger awkward encounter as shouting 'Pirate Booty' is misinterpreted making for a fanfiction.net moment with the animatronic which seem to assume the spirits grew up since their capture within the suits, so have their perception of their 'self'. [Mangle will appear to those that selected preference Female, Foxy to everyone else upon awkward trigger.]] 

--Out of Order-- At random, should you choose to go to Pirate Cove and the sign is out front, your not allowed to enter. Darn sign!

Office:

-Flashlight! [Shine the flashlight randomly, hoping to use the A.I. glitch against the haunted moving parts. Chance of not shining it in the right place, Very Rare Chance of triggering Golden Freddy illusion to eject you from the event. Small chance you will think 'Fleshlight' instead and either trigger awkward moment or suffer pacify/skip a turn from an animatronic due to not using the flashlight.]

-Watch the Cameras! [Determine how many cameras you want to skim through to see where everyone is, the more you choose to skim through, the higher chance you are at of being attacked by an animatronic. 

-Squeak Nose on Poster [You squeaked a interactive nose toy on a poster. Did you think this would help?]

-Search for Gun [Thinking your a guard deserving of actually being able to protect yourself, you look for a gun. Small chance gun you use for duel if found this way is a well made Water Gun]

-Look Through Vents [Chance of getting attacked, chance of finding item, very rare chance of going through a complicated section of the vents and being ejected to the regular map.]

Supply Closet:

-Search for Supplies [You don't know what your looking for, but your hoping you find something useful. If attacked in Supply Closet you were cornered so you suffer pacify and skip a turn.]

Kitchen:

-Look for Pizza [You attempt to get food to sate your hunger in this scary ordeal, if found Pizza goes to inventory.]

-Look for Soda [You need a new drink as the one in the office is empty...]

-Attempt to fix Camera [Sacrifice mechanical object to see if the parts you use will help it run again, doesn't always work. Camera is audible and can thank you by telling you the location of one thing you ask to find in the Kitchen not on the initial list, like a Golden Freddy piece.] 

This is a small portion of ideas and options...I know my idea could be broken for a number of reasons but that's why I posted it on this forum, so it could be refined by others with better ideas.