Achtung, Spy!

A social deduction game originally inspired by the idea of a Resistance-style game where instead of pass/fail, the missions pit you against a symmetrical, opposing team. Themed around Team Fortress 2, the aim of the game is for each team to select a squad that will beat the other team's squad; each role has different advantages and disadvantages depending on the match (mission) being played. However, each team has a Spy on it who will do everything they can to sabotage their team. Default playercount is 10, with another person required to act as The Administrator (handling the results of actions so as to keep things secret from all players); as such it may be best played online (via IM or over a forum)

DISCLAIMER: This game is entirely untested. All elements of the game are prospective and may not work as expected. If you do give it a shot, please let me know how it pans out!

Setup

The ten players are separated into two teams of five at random, a RED team and a BLU team. Four random, unique, non-Spy classes (described below) are then selected. The Spy class is then added to this list; this is the class pool. Each player on each team then receives a different one of the classes in the pool. Finally, a player from each team is selected at random to start out as Team Captain and given an unusual, well, Team Captain (it's a hat, you heathen). An order is selected at random for the other players to receive the hat (and less importantly, the mechanical effects). The game now progresses in rounds, as follows:

A round

The current match is selected at random from among three options: Payload, Capture the Flag, and Control Points.
Teams discuss which three players they would like to form the squad which will be participating in this match.
The Team Captain nominates a squad of three to participate in the match.
Teams discuss the proposed squad, then each individual member votes yes or no. A majority vote of yes locks in the squad to participate in the match. A majority vote of no means another round of discussion and voting. Either way, the Team Captain passes their hat to the next player in order.
If a player would pass the Captain's hat to someone who has already had it in this voting cycle, they still do so but their squad is automatically approved and no voting takes place.
The match is now played out: each team's points are added up, and the team with the most points for the type of match selected receives a win. A tied match results in half a win to both sides.

Ending the game

Once five matches have been played, the team with the most wins is victorious.

Classes

Each class has a different point value for each type of match, or a special ability. The classes are as follows:

Scout - Payload 1, Capture the Flag 3, Control Points 2.
Soldier - Payload 2, Capture the Flag 1, Control Points 3.
Pyro - Payload 3, Capture the Flag 2, Control Points 1.

Demoman - Payload 1, Capture the Flag 2, Control Points 3.
Heavy - Payload 3, Capture the Flag 1, Control Points 2.
Engineer - Payload 2, Capture the Flag 3, Control Points 1.

Medic - Choose a friendly player who is in the match. Double their point value. Will automatically switch to the other squad member if their target is removed.
Sniper - Choose an enemy player who is in the match. That player is removed from the match. Will fail if aimed at a Spy; friendly fire is off.
Spy - Wins with opposing team. May choose one of the following actions to perform each match, after the squad is locked in but before the match takes place:

Backstab: Choose a friendly player who is in the match. That player is removed from the match. May only be activated if the Spy is in the match.
Sabotage: Choose a friendly player. Attempt to guess their class; if successful, the Spy is notified and the target's point values are reduced by 1 for the rest of the game. A sabotaged Sniper will instead only decrease the target's point value by 1 (this occurs before Medic doubling) while a sabotaged Medic will instead only increase the target's point value by 1.

All actions are performed privately by informing the Administrator of targets. A removed player contributes no points and fails to activate their abilities, if any; notably, a Sniper who is backstabbed will fail to activate. A sabotaged player is privately notified of the sabotage the first time they are locked in as part of a squad, after the match concludes. A player may only be sabotaged once, and sabotage has no effect on spies.

Setup decisions

There are a few decisions to make about what information the players get in addition to the bare minimum of match results. They are as follows:

Whether or not to announce that a player was removed from a match. If yes, whether or not to announce the cause of the removal (i.e. "Redmond was backstabbed!" vs. "Redmond was removed!"). Note that a failed removal attempt is NEVER announced publicly. Announcing removals and especially causes makes it more obvious when a Spy has infiltrated a squad.
Whether or not to announce the point totals after a match is over, rather than just declaring a winner. Announcing point totals provides more information about which players have which classes.
Whether or not to make the players aware of which classes are in the class pool. Making the players aware of this makes it harder for the Spy to pretend to be another class, but also makes it harder for the other players to avoid being sabotaged.

The above rules are sufficient to play a full game. Below are additional options to change your playing experience if desired (WARNING: This stuff is even more experimental and potentially bad than the base game).

Optional rules

These extra rules can be used to change up the playing experience. They are recommended only after you have already tried a few games using the base ruleset.

Rather than selecting a match type at random, each player casts a vote for what type it will be; Administrator decides whether the vote is public or private. This adds an extra decision-making dimension.
Rather than selecting a match type at random, the Administrator decides before the game starts what each match will be. This information may be public or private.
Rather than the Medic choosing a target, they automatically target the best possible player. This streamlines the game and removes an action, but removes a decision element as well. The Medic does not learn who they targeted.
The teams have different class pools. This may cause the game to become unfair.
The class pools are chosen by the Administrator at the start of the game. This is good for creating a specific type of game.
Each player except the Spies receives a new class after every match. Sabotage only works until a player changes classes. This makes information gathering harder and de-emphasizes the importance of class reads.
If the above is selected, class pools may be changed at specific points in the game, either at random or by the Administrator. This is also good for creating a specific kind of game.
After squads are locked in, Spies may write a short note which will be received by the other team, and/or teams may vote for a player to write a short note which will be seen by the other team's Spy. This allows the Spies to actually communicate with the team they are winning with, albeit in a limited fashion, and may be useful for informing Snipers. It also allows the Spy's real team to help them figure out what classes their "teammates" are.
If point totals are being revealed after each match, give the Spy the following additional action option:

Disguise: The Spy chooses a class and appears as that class when point totals are revealed. This does not affect the match outcome; if RED team has more points than BLU team but loses or draws, this indicates that a Spy was present on RED team.

Change the player count. This may require changing the number of spies and/or squad sizes. Multiple spies may or may not be made aware of each other. Medics are permitted to provide a target priority list if joining a squad size of greater than 3; if their target is removed, they will move down the list to the next one. Alternatively, new targets may be chosen at random, or automatic optimal targeting may be used. Recommended numbers for different player counts are listed below:

4 players per team: 1 spy, squad size 2
5 players per team: 1 spy, squad size 3
6 players per team: 2 spies, squad size 3
7 players per team: 2 spies, squad size 3
8 players per team: 2 spies, squad size 4
9 players per team: 3 spies, squad size 4
10+ players per team: Increase the spy count at multiples of 3, and the squad size count at multiplkes of 3 minus 1. Also stop being crazy.

If there are 9 players per team: Set the class pools to one of each class, and the squad size to 6. Besides being classic TF2, this shifts the emphasis towards sabotaging and effective use of the Medic and Sniper to gain an edge over the opposing team.

More complex classes

These class stats may be used instead of the original ones for more complex gameplay.

Scout - Payload 2, Capture the Flag 3, Control Points 3.

-1 to all stats if targeted by the Medic. This effect takes place before stat doubling.

Soldier - Payload 2, Capture the Flag 2, Control Points 2.

+1 to all stats if targeted by a Medic. This effect takes place before stat doubling.

Pyro - Payload 1, Capture the Flag 1, Control Points 1.

If on the same squad as a Spy, the Spy is removed from the squad and their ability is negated.

Demoman - Payload 1, Capture the Flag 2, Control Points 3.

All Engineers on the enemy squad receive -1 to all stats.

Heavy - Payload 3, Capture the Flag 1, Control Points 2.

+1 to all stats if targeted by the Medic. This effect takes place before stat doubling.

Engineer - Payload 3, Capture the Flag 3, Control Points 3.

-1 to all stats if a Spy is in the same squad. Unaffected by Medics.

Medic - Choose a friendly player who is in the match. Double their point value. Will automatically switch to the other squad member if their target is removed.

Ubercharge: On the third match a Medic participates in, the Medic and their chosen teammate cannot be removed except by a Spy and the squad receives +2 points. A sabotaged Medic cannot Ubercharge.

Sniper - Choose an enemy player who is in the match. That player is removed from the match. Will fail if aimed at a Spy; friendly fire is off.
Spy - Wins with opposing team. May choose one of the following actions to perform each match, after the squad is locked in but before the match takes place:

Backstab: Choose a friendly player who is in the match. That player is removed from the match. May only be activated if the Spy is in the match.
Sabotage: Choose a friendly player. Attempt to guess their class; if successful, their point values are reduced by 1 for the rest of the game.

Playing a physical game (WIP)

This space reserved for advice on how to play the game in a real world setting without the benefit of people being able to easily privately message each other.

Unlocks (WIP, highly experimental, probably hilariously unbalanced)

There is no specific use for these; they may be start of game modifiers, or classes such as "Jarate Sniper" may be included in the list of possible classes, or individual players may acquire them openly or secretly under certain conditions. Use them as a balancing tool if your playgroup finds certain things to be unfun, or just to add extra complications if desired.

Scout

Bonk! Atomic Punch - The Scout cannot be removed from a match.
Crit-a-cola - +1 in Control Point matches.

Soldier

The Pain Train - +1 on Payload and Control Point matches, but -1 on Capture the Flag matches.
The Half-Zatoichi - A single player with the Half-Zatoichi is chosen at random; all other players with the Half-Zatoichi are removed.

Pyro

The Backburner - +1 in all matches, -1 in all matches for each Soldier or Demoman on the enemy squad.

Demoman

The Eyelander - +1 in all matches for each match the Demoman has won.
The Pain Train - +1 on Payload and Control Point matches, but -1 on Capture the Flag matches.
The Half-Zatoichi - A single player with the Half-Zatoichi is chosen at random; all other players with the Half-Zatoichi are removed.

Heavy

Natascha - All Scouts on the enemy squad receive -1

Engineer

The Gunslinger - No longer receive -1 from Demomen and Spies, Medics now grant +1 instead of being ignored (Sabotaged Medics are still ignored).

Medic

The Kritzkreig - Ubercharge now grants +4 points, but does not provide immunity to removal.
The Ubersaw - Ubercharge now occurs on the second and fourth matches the Medic participates in.

Sniper

The Razorback - Cannot be removed from a match by a Spy unless sabotaged. Is automatically sabotaged when a Spy attempts to remove them from a match.

Spy

The Ambassador - The Spy receives the following extra ability. [Headshot: Choose an enemy player. If that player is a Spy, remove them from the match and negate their ability. If multiple spies successfully use Headshot, resolve them all, then resolve other remaining spy abilities]

Maps (WIP, highly experimental, probably hilariously unbalanced)

None of these exist yet, but the idea is to select one along with match type; the map then modifies the match in some way.