Thanks to Half-Life 2: Raising the Bar and the Half-Life 2 beta leak from 2003, we've had a few little looks at concepts Valve considered for the FPS sequel but had to cut during development. For instance, thanks to files found in the leaked version, we know there were plans for a level set on an icebreaker ship that would lead to a mini-submarine trip to an underwater base—an idea that ended up being re-used in the Half-Life 2: Episode 3 synopsis, and the fan game based on it.
The recent 20th anniversary documentary has shone even more light on the earliest plans for Half-Life 2. Interestingly, those early plans weren't doing much to build on the plot of the first game. "The first two years we tried a lot of things," says writer Marc Laidlaw, "and a lot of those story arcs that we had initially didn't have really any carryover from Black Mesa or any of that."
