Team Fortress 2 (partially found original builds of PC game; 1999-2006)
Team Fortress 2 (abbreviated to TF2) is a free-to-play class-based first-person shooter game developed by Valve Software, the company behind Team Fortress Classic and the critically acclaimed Half-Life franchise. Originally being a mod for Quake, Team Fortress 2 has been well received for its solid and innovative gameplay with an incredibly active community that garners more than 50,000 players per day. Since production began back in 1998, the game has gone through many different iterations that were radically different from the final released version. According to the Team Fortress team, the art style alone went through numerous phases before finally settling on the design used today. 
Team Fortress 2 began development in 1998 before the release of Team Fortress Classic. Originally entitled "Valve's Team Fortress", the game was originally planned to be a much more realistic shooter with large maps, usable vehicles, and elements taken from real-time strategy games.  According to an interview with Robin Walkers, there were originally 8 different classes that were reminiscent of the original classes but still unique and different from their older counterpart. The Pyro class was also removed for unspecified reasons.
This game was later renamed to "Team Fortress 2: Brotherhood of Arms" in 1999 alongside with more features being introduced.
Brotherhood of Arms (1999) / Valve's Team Fortress (1998)
The game would've featured a unique class called the "Commander". The Commander has the ability to see the entire battlefield and the Engineer's cameras, issue commands, establish objectives, and monitor the player's stats. It's the only class in the game that has this ability. Another uniquely different class was the "Medium Infantry", which was a temporary class used by the Engineer and Sniper.
The original classes have also gained a few unique abilities. The Engineer keeps his original buildings from before but gains a regular camera, a security camera, and a drill motor. Rocket Infantry is the Soldier class but with a jetpack instead of being able to rocket jump. The Sniper can go completely invisible for twenty seconds if he learns the "Fatal Breeze" ability, making it much more frustrating than ever before. The Light Infantry is the Scout class with the only difference being able to use light machine guns. Field Medic can heal players by looking at them for just a second or two. The Spy is similar to its original counterpart. And the Commando being a prototype of the Demoman, equipped with only a grenade launcher.
Around E3 1999, the game was later renamed to Team Fortress 2: Brotherhood of Arms. The game kept the same ideas and concepts shown in the 1998 preview but with the plans of new maps that held unique objectives, adding addition weapons like the Smoke and Spider Grenades, reintroducing the Pyro class, and adding new classes like the Ranger, Machine Gunner (which replaced the Medium Infantry class), and the Instructor. 
According to a 1999 interview on Boot Disk, the game's campaign was originally going to be a cross between singleplayer and multiplayer.
Campaigns are a series of maps where outcomes and objectives you have completed in the first of subsequent maps may affect the situation in later maps. Which means that you might have to achieve this objective on the first map but if you achieve these two side objectives on the first map and in the second map, you might have some advantages over your enemies. So the idea is you play through an entire campaign of maybe three of four maps with like (say in the clan situation and it's not necessarily the first or the last map winning that wins you the match) it's how well you do over the entire campaign. And so you might invest resources in your first map to achieve something just so you've got a better situation in the later map.
In the Saving Private Ryan campaign, it's a series of three maps where in the first map, one team's storming the beach while the other team is holding the beach. The first time has to simply get past the beach and reach a safe zone and they get points for every guy they push past the safe zone. And the defending team gains points for how far back they keep the attackers. In later maps, the attacking team has more lives depending on the number of guys they've pushed through in the first map. - Robin Walker, circa 1999. 
In the same interview, Robin is asked about vehicles, the usage of vehicles within the main campaign, and how they would affect the overall gameplay. Rob states that vehicles will be used for everything but driving. Essentially they were gonna be used for things like covering from fire and even used for deploying onto the battlefield. Some vehicles (like the tank) would be used to target enemies, call an Apache helicopter to attack specific enemy positions, or even call in support. 
Sadly, there is barely any concept art available to the public. The only known concept art can be found on the Team Fortress 2 wiki.
The game was ultimately scrapped later on in development, but some leftover functionality and mechanics were later reused in Team Fortress Classic.
Team Fortress 2: Invasion (2003)
Unlike its predecessors, Invasion would take a less realistic approach and sought for a steampunk and sci-fi design with a war between humans and aliens but still keeping most elements from Brotherhood of Arms intact. The only reason for knowing the game's existence was a build leaked in 2003. It was theorized by many that it would
Early in development, the team scrapped a lot of older code from the game due to conflicting designs, but surprisingly enough, it still kept classes like the "Commander". Another major difference is the lack of planned classes appearing in the game. There were only a handful of new classes added to the game. One of them being known as the "Escort". It was essentially the Heavy weapons class but with a more concrete focus on defense. Another class addition is the "Sapper", which can produce sappers and a weapon that drains enemy's health. The other main classes would be given different names like the Soldier being named "Commando", Engineer being named "Defender", Spy being named "Infiltrator", and many more. And the other new class is simply referred to as "Support". Which has the ability to throw mortars and flashbangs.
The average round would consist of 15-minute intervals called "Acts" that pinned two teams (Aliens and Humans) fighting against each other. It is assumed that the aliens act as the attackers while the Humans act like the defenders. Each team has their own key differences. With the humans, buildings require a lot of power for their buildings but costs less to build. While the aliens don't require any power but buildings cost more as a result. The teams essentially have to control territory known as "Control Zones". The one with the most control zones by the end of the round wins. 
It is assumed that a year or so afterwards, this idea was scrapped completely. The source code of the build was found within the 2003 leak alongside with some temporary models and textures.  Some people have attempted to make a recreation of what the game might've been like with the leaked source code.
Final Design (2006)
Years after anticipation, Valve re-announced Team Fortress 2 in 2006 alongside with the orange box. In July of the same year, Electronic Arts and Valve Software held a conference revealing the final design of the game. The two trailers that were revealed contained a build from the Beta version of Team Fortress 2 which also contained some early designs of the classes.
A lot of the ideas presented in the Brotherhood of Arms and Invasion were scrapped entirely. It is uncertain why they decided to go for this, but it is assumed that the gameplay was too complex and far off from the original Team Fortress mod. Classes like the "Commander" and such were completely removed from the game. But the original classes and their old abilities were retained through this final iteration. And unlike the previous versions, the overall design for the game is less realistic and resembles more of a cartoon from the early 50s.
Despite the overall design being decided, many of the designs and items used in the trailer were altered or completely removed prior to the official release. For example, the Heavy originally did not have any gloves on his model and the Scout used the SMG instead of his iconic scattergun. The Spy was allowed to use a tranquilizer dart gun and had an armband on his left arm. Another example is the Soldier's rocket launcher having a rocket on the muzzle and the engineer having a needle as a melee weapon instead of a bonesaw. One noticeable change from this build is 2fort's map. Unlike in the current version, 2fort lacked a roof on the bridge and the spawn doors near the courtyard had an entirely different layout.
There are also a bunch of features and items removed from the game. One notable feature being the grenades. Each class would be allowed to use specific grenades like the Pyro being able to use a "Napalm" grenade. But due to how unbalanced the grenades were, they were ultimately scrapped from the game. A few other items that were removed from the game are Medic's syringe gun and the Spy's tranquilizer.
Currently, there is no available build from 2005-2006. There have been numerous mods attempting to recreate the old beta builds, but there has been no confirmed discovery of the builds.
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